Epoxidized Natural Rubber-Derived Carbon Nanofibers with Tunable Microporosity for Efficient CO₂ Capture

Abstract

This study developed porous carbon nanofibers (CNFs) to enhance the carbon dioxide (CO₂) uptake capacity and adsorption kinetics. The CNFs were fabricated via electrospinning of a polymeric precursor consisting of epoxidized natural rubber (ENR) with polyacrylonitrile (PAN), followed by subsequent thermal treatment. ENR contents ranging from 10 to 80 wt % were incorporated into PAN to investigate their effects on the textural properties and porosity development of CNFs produced by electrospinning. The influences of textural characteristics, physical activation, and adsorption kinetics on the CO₂ uptake capacity were systematically investigated. Among the prepared samples, 20ENR-CNF exhibited the highest CO₂ uptake capacity, achieving 3.53 mmol g^–1 for the as-prepared sample and 4.35 mmol g^–1 after activation at 273 K under 1 bar. This enhanced performance is attributed to its high porosity and favorable isosteric heat of adsorption (28.18 kJ mol^–1). Kinetic studies demonstrated that varying the ENR content significantly affected both the CO₂ uptake capacity and adsorption rate, with intraparticle diffusion identified as a key controlling mechanism. ENR incorporation played a crucial role in tailoring the pore structure within the CNF matrix, thereby improving the CO₂ adsorption performance. A pressure swing adsorption test conducted over five cycles confirmed the excellent recyclability of the activated 20ENR-CNF sample. Furthermore, increasing the ENR content was found to significantly enhance the CO₂ adsorption rate.

1. Introduction

Carbon dioxide (CO₂) emissions have become an urgent global environmental and public health concern. Large amounts of CO₂ released into the atmosphere have led to severe environmental consequences, including climate change and global warming. , To mitigate these effects, many countries have committed to achieving net-zero CO₂ emissions by 2050, prompting intensive research into the development of new sorbent materials for CO₂ capture. − Among the various carbon capture and storage (CCS) strategies, adsorption is considered one of the most promising approaches due to its operational simplicity, high efficiency at low CO₂ concentrations, and excellent recyclability. , Compared with conventional amine-based adsorption technologies, adsorption offers several advantages, including lower energy consumption, noncorrosive operation, and the absence of water in the operation process. , Alternative solid adsorbents, such as activated carbon, , carbon fibers, − zeolites, , zeolitic imidazolate frameworks (ZIFs), − and metal organic frameworks (MOFs), , have demonstrated considerable potential for CO₂ capture as substitutes for amine-based sorbents. Moreover, the synergistic effect of heteroatoms, such as boron, nitrogen, and oxygen, significantly enhances the CO₂ adsorption performance of a porous carbon surface. − These materials are regarded as cost-effective options due to their high CO₂ uptake capacities, good selectivity, strong adsorption performance, and relatively low regeneration costs.

Low-cost porous carbon materials have attracted significant attention because they can achieve high CO₂ uptake capacities with maintaining good regeneration stability. − Among these materials, carbon nanofibers (CNFs) are particularly promising for CO₂ capture owing to their high surface area and the availability of abundant polymer precursors. CNFs possess tunable porous structures and favorable physical properties, often outperforming many conventional adsorbents in CO₂ adsorption performance. ,− CNFs can be fabricated from various polymer precursors via electrospinning, a versatile and continuous technique that produces nanoscale polymer fibers, which are subsequently carbonized through heat treatment. The surface structure and physicochemical properties of CNFs strongly depend on the type of polymer and the composition of the precursor solution. Polyacrylonitrile (PAN), a polymer with high carbon yield, is widely used as a precursor for CNF fabrication due to its excellent spinnability and high carbonization efficiency.

Porous CNFs have attracted considerable interest as CO₂ adsorbents due to their tunable pore structure, high surface area, and structural stability. However, most reported CNF systems rely predominantly on PAN-based precursors and focus mainly on increasing surface area through physical or chemical activation, while the role of sustainable pore-directing agents in tailoring pore architecture and adsorption kinetics has received comparatively less attention. For example, PAN-based carbon nanofibers electrospun with polyvinylpyrrolidone (PVP) generated rough and extremely porous nanofibers, which, after carbonization, developed multiaperture/opening porous structures with ultrasmall pores and an average pore diameter of approximately 0.71 nm. These porous carbon nanofibers (PCNFs) exhibited high CO₂ selectivity and a CO₂ adsorption capacity of 3.11 mmol/g. Similarly, poly­(methyl methacrylate) (PMMA) has been used as a sacrificial polymer in PAN precursor solutions to fabricate electrospun nanofibers. Upon thermal treatment, PMMA decomposition generated abundant micropores, resulting in a high surface area of 884 m² g^–1 and a CO₂ uptake capacity of 3.16 mmol g^–1 at 1 bar and 298 K. In addition, the incorporation of nitrogen-rich organic compounds, such as melamine blended with a PAN precursor, has been reported to enhance microporosity and specific surface area. After carbonization, the resulting CNFs exhibited an increased specific surface area (547  m² g^–1) and micropore volume (0.220 cm³ g^–1). Under optimal melamine doping conditions, the CNFs achieved a maximum CO₂ adsorption capacity of 3.15 mmol g^–1.

In particular, the use of biobased elastomeric polymers such as epoxidized natural rubber (ENR), derived from renewable natural rubber, offers a sustainable alternative due to its flexible molecular structure, unique thermal decomposition behavior, and potential to generate tailored porosity during carbonization. Incorporating ENR into PAN-based electrospinning systems may serve a dual function: acting as a pore-generating phase while simultaneously modulating fiber morphology and pore connectivity. However, systematic investigations into the influence of ENR content on textural properties, micropore development, adsorption thermodynamics, and CO₂ adsorption kinetics remain limited. Furthermore, few studies have established clear correlations between pore structure evolution, kinetic adsorption behavior, and recyclability under pressure swing conditions. Addressing these gaps is essential for the rational design of sustainable CNF adsorbents with enhanced CO₂ uptake capacity and fast adsorption kinetics for practical carbon capture applications. Therefore, this study aims to elucidate the role of ENR as a sustainable pore-directing agent in the fabrication of porous CNFs and to establish the structure–property–performance relationships governing CO₂ uptake capacity, adsorption kinetics, and cyclic stability.

2. Experimental Section

The chemicals used in the experiment were commercially obtained and used without further purification/processing. Polyacrylonitrile (PAN, cal. MW = 150,000) was obtained from Sigma-Aldrich. N,N′-Dimethylformamide (DMF) was from Carlo Erba Reagents. Hexanes were obtained from Macron Fine Chemicals. Raw natural rubber cup lump (NR) was supplied from an agricultural farm in Thailand. Ammonia solution (30% w/v) was obtained from Loba Chemie and hydrogen peroxide (Assay 35%) was obtained from QRëC New Zealand. Nitrogen gas 99 999% (UHP), carbon dioxide gas 99.999% (UHP), and helium gas (99.999%) UHP were supplied by S.I. Technology Co., Ltd., Thailand.

2.1. Preparation of Porous Carbon Nanofibers

2.1.1. Preparation of Epoxidized Natural Rubber

In this study, epoxidized natural rubber was prepared by the epoxidation process, following previous methods. − The NR was cut into small pieces and dried at room temperature for 1 week. Then, 2 g of dried NR was dissolved in 20 mL of hexane for 12 h until a latex solution was obtained. Next, 20 mL of formic acid was slowly poured into the latex solution with continuous stirring for 6 h. During this step, the white latex solution turned a pale-orange color. Excess peroxide was then added under reflux at 353 K for 4 h. Finally, ammonia solution was gradually dropped into the epoxidized NR solution, followed by slowly poured cool distilled water, to obtain the white precipitated form of the ENR product. The precipitate was filtrated and dried at room temperature to reduce the water content. The dried ENR powder samples were kept in a desiccator cabinet to prevent moisture reabsorption.

2.1.2. Preparation of nENR-CNF Samples

The polymeric electrospinning technique was used to fabricate the as-spun mixed polymeric fibers, with conventional heat treatment applied before the CO₂ gas activation process, as illustrated in Figure . The CNF samples were prepared by using mixed polymeric precursor solutions. Ratios of 10, 20, 30, 50, 70, and 80 wt % ENR to the PAN polymeric precursor (15 wt %) were dissolved in DMF under vigorous stirring for 12 h at room temperature to obtain homogeneous solutions. These polymeric solutions were then subjected to electrospinning to fabricate electrospun fibers, using a stainless steel gauge needle with a 1 mm inner diameter, connected to the anode of a DC power source at 15–18 kV and a solution flow rate of 0.05 mL h^–1. The cathode current collector was a stainless steel roller wrapped in aluminum foil, with 15 mm from the tip to the collector. The obtained fibers were stabilized under 533 K for 2 h with a heating rate of 1 K min^–1 and then carbonized at 1173 K for 2 h with a heating rate of 10 K/min under a nitrogen atmosphere to obtain porous carbon nanofibers. The samples were kept in a desiccator cabinet and labeled as nENR-CNF, with n representing 10, 20, 30, 50, 70, or 80 wt % ENR. The 80ENR-CNF sample presented a significant obstacle for large-scale fiber production because of its low electrical conductivity and high ENR content. Moreover, owing to its insulating nature, the polymeric precursor becomes stuck at the tip, causing the Taylor cone to flicker. Therefore, the 80ENR-CNF sample was prepared for the fundamental structural and physical characterization.

1.

Schematic illustration of the preparation of activated nENR-CNFs via electrospinning and CO₂ gas activation.

2.1.3. CO₂ Activation of CNF Samples

The prepared CNF samples were activated by using CO₂ gas at 1173 K. The nENR-CNFs were placed in a quartz tube with a continuous flow of CO₂ (20 mL min^–1). Heat was applied from room temperature to 1173 K at a heating rate of 5 K min^–1, with activation at 1173 K for 2 h. The obtained activated nENR-CNFs were kept in a desiccator before the measurement of CO₂ adsorption.

2.2. Characterization

The chemical functional groups of the as-spun fibers were analyzed using Fourier transform infrared spectroscopy (FTIR) with a Bruker INVENIO FTIR spectrometer (Bruker, Germany). Data were collected using an attenuated total reflection (ATR) technique connected to infrared spectroscopy in the range of 400–4000 cm^–1 with 16 scans. Thermal stability and weight loss of the as-spun fiber samples were determined by thermogravimetric analysis (TGA) using a TGA2 STARe system (Mettler Toledo, Switzerland) performed in the range of 313 to 973 K with a heating rate of 10 K/min under a nitrogen flow of 5 mL min^–1. The crystal structure of the samples was characterized using Raman spectroscopy (RS) (HORIBA KH-8700, HORIBA, Japan) and analyzed by powder X-ray diffraction (XRD) using a D8 advanced X-ray diffractometer (Bruker, Germany) equipped with a Cu Kα sealed-tube X-ray source (λ = 11.5418 Å). The surface morphology of the carbon fiber samples was examined using a field-emission scanning electron microscope (FESEM) (Merlin Compact, Zeiss, Germany), with images acquired under an in-lens duo detector system, employing in-lens secondary electron (SE) and energy-selective backscattered (EsB) detectors. Gas adsorption–desorption experiments up to 1 bar were measured using a Quantachrome Autosorb iQ2 automated gas sorption analyzer (Quantachrome Instruments, USA).

2.3. Adsorption Measurements

The textural surface of the samples was determined by N₂ adsorption–desorption isotherms measured at 77 K using an Autosorp iQMPXR (Quantachrome). Before measurement, the samples were outgassed at 523 K for 8 h under vacuum. The specific surface area (SSA) of the samples was measured in P/P₀ using the Brunauer–Emmett–Teller (BET) and Langmuir methods, with the micropore (<2.0 nm) surface area (S _micro) obtained using the T-plot method. The single-point total pore volume (V _t) was measured at P/P ₀ = 0.995, while the mesopore volume (V _meso) and micropore volume (V _micro) were calculated using a nonlocal density function theory (NLDFT) model. Pore size distribution was obtained from the Barrett–Joyner–Halenda (BJH) method. The CO₂ adsorption isotherms were measured on an Autosorb iQMPXR instrument (Quantachrome) at 273 and 298 K. Approximately 50 mg of samples was outgassed at 523 K for 8 h under vacuum and a He gas environment before the measurement. The adsorption isotherms were recorded at 1 bar while flowing ultrahigh purity of CO₂ gas. Moreover, the cyclic adsorption–desorption performance was conducted by CO₂ adsorption at 298 K for 60 min, followed by N₂-assisted desorption during heating to 398 K using thermogravimetric analysis (TGA2 STARe, Mettler Toledo, Switzerland).

2.4. Adsorption Kinetics

Pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD) models were used to measure the CO₂ adsorption rate and the performance of the adsorbent, with rate constants evaluated using eqs , , and .

Pseudo-first order (PFO):

$$\ln (q_e-q_t)=\ln q_t-k_{1}t$$ 1

In order to fit the kinetic data and evaluate the parameters of q _e and k ₁, eq has been fundamentally used by plotting ln­(q _e –q _t ) versus t, where k ₁ is the rate constant of the PFO model, q _e is the adsorption capacity at equilibrium, and q _t is the amount of CO₂ adsorbed at time t.

Pseudo-second-order (PSO):

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{1}{q_e}t$$ 2

The PSO models based on adsorption equilibrium capacity, eq , have been commonly used by plotting t/q _t versus t. where k ₂ is the rate constant of the PSO model. The intercept of the plot helps determine the PSO rate constant (k ₂).

Intraparticle diffusion (IPD):

$$q_t=k_{id}{t}^{1/2}+C$$ 3

where k _id is the intraparticle rate constant, determined from the slope of the q _t versus t ^1/2 plot, and C is the intercept that represents the boundary layer thickness.

3. Results and Discussion

3.1. Characterization of As-Spun ENR-PAN Fibers

FTIR spectroscopy was employed to monitor the epoxide group formation in the NR structure and confirm the oxidized functional groups on the ENR precursor. Figure (a) shows the FTIR spectra of NR (green spectrum) versus ENR (red spectrum). Both spectra presented vibrational frequencies in the range 2985–2852 cm^–1 and strong absorbances at 1446 and 1376 cm^–1 that were assigned to the stretching and bending vibrations of methyl (–CH₃) groups, respectively. The medium peak at 1660 cm^–1 and the strong peak at 840 cm^–1 of the green spectrum were assigned to –CC- stretching and –CH₃ bending (disubstituted alkene) of the NR structure, respectively. After oxidation, the ENR spectrum showed vibrational frequency at 1722 cm^–1, which was assigned to the stretching of the carbonyl group (–CO). The peaks at 1172 and 1066 cm^–1 were assigned to characteristic C–O–C stretching signals, implying that epoxide groups formed on the NR structure. The absence of the disubstituted alkene signal in the ENR spectrum was due to the alkene group changing to epoxide. ,

2.

IR spectra of (a) the natural rubber (NR, green line) and the epoxidized natural rubber (ENR, red line) and (b) different contents of as-spun nENR-PAN fibers.

All the as-spun ENR-PAN fibers were subjected to FTIR spectroscopy to observe the changing effects of the ENR contents. Figure (b) presents the FTIR spectra of the as-spun polymeric fibers. All the nanofibers showed the characteristic peaks of –CH₃ stretching and bending vibrations from 2977 to 2854 cm^–1 and 1452 to 1373 cm^–1, respectively. The characteristic peak of the cyanide (–CN) group in the PAN structure was displayed as a strong and sharp peak at 2244 cm^–1. The strong intensities around 1074–1043 cm^–1 were assigned to the epoxide group signal from the ENR precursor, implying that the prepared ENR polymer was mixed with the PAN polymer during electrospinning to produce copolymeric fibers.

The graphitic regions and the carbon phase for all the nENR-CNF samples were examined by XRD and Raman techniques to determine the carbon nanofiber structure. The XRD patterns are depicted in Figure . A major diffraction peak appeared at 25°, corresponding to the amorphous structure of typical carbon patterns and confirming the conversion of the polymeric fiber into carbon. The characteristic diffraction peaks at 25° (002) and 43° (101) are related to amorphous and graphitic carbon in the nanofiber samples, respectively, concurring with previous reports , and indicating good carbon fiber formation after the carbonization process.

3.

Powder patterns of the nENR-CNF samples.

The as-spun fibers were stabilized and carbonized at 900 °C, and all the nENR-CNF samples were analyzed by Raman spectroscopy (Figure ). The results confirmed that high thermal treatment successfully transformed the copolymer (PAN and ENR) into the carbon form. The two significant peaks at the D-band (1340 cm^–1) and G-band (1590 cm^–1) represented the disordered and graphitic carbon structure, respectively. All the samples showed I _D/I _G ratios in the range of 1.00–1.06, implying that the polymeric fibers were converted to the carbon form. The consistency of the Raman spectra and XRD patterns confirmed the formation of the carbon structure.

4.

Raman spectra of the nENR-CNF samples.

Figure shows the microscopic images at a high magnification of as-prepared nENR-PAN CNF samples with diameter size distribution. The scale bar indicates a length of 1 μm. The surface morphology of the carbon nanofibers showed a randomly distributed diameter size and surface features. The 10, 20, and 30ENR-CNFs had smooth surfaces and were less merged, whereas the 50, 70, and 80ENR-CNF samples presented rough surfaces and were merged together. This implied that adding more ENR impacted the electrospun fiber formation during the electrospinning process. When the ENR content increased, the carbon fibers were highly merged between the outer surfaces. CNFs with ENR at more than 30 wt % exhibited wavy and curled fibers, forming an interconnected network, resulting from their nonuniform growth orientation during the fabrication process. The sticky and curled carbon fibers were influenced by the interfacial interaction of ENR during the electrospinning process. , The 80ENR-CNF exhibited the largest diameter size and interconnected morphology (Figure (f)), with fiber joints visible at the intersections. The average diameter of the carbon nanofibers was 50ENR-CNF (592 nm) < 70ENR-CNF (641 nm) < 30ENR-CNF (700 nm) < 10ENR-CNF (895 nm) < 20ENR-CNF (1152 nm) < 80ENR-CNF (1170 nm). These samples had random diameter size, suggesting that diameter size did not depend on the amount of ENR but was possibly impacted by the lower electrical conduction of the ENR precursor.

5.

SEM images and the diameter size distribution of (a) 10ENR-CNF, (b) 20ENR-CNF, (c) 30ENR-CNF, (d) 50ENR-CNF, (e) 70ENR-CNF, and (f) 80ENR-CNF samples.

The N₂ adsorption–desorption curves of all the activated nENR-CNF samples are presented in Figure (a), and the textural properties of the samples are shown in Table . All the samples exhibited type-I class isotherms as defined by IUPAC, which are characteristic of a narrow micropore size distribution. The results implied that micropores were formed during the carbonization process because of the decomposition of some parts of the polymeric precursor. The pore size distribution curves depicted in Figure (b) provide additional insight into the CNF structure. All carbon nanofiber samples show various pore sizes. The 20-70ENR-CNF samples exhibit microporosity (<2 nm), while the 20ENR-CNF and 30ENR-CNF samples also show pore sizes in the range of approximately 2–6 nm, confirming the dominance of mesopores.

6.

(a) N₂ adsorption–desorption isotherms and (b) pore size distribution of the activated nENR-CNF samples.

1. Textural Properties from N₂ Adsorption–Desorption Isotherms for Activated nENR-CNF Samples.

samples	SSABET (m2 g–1)	SSALangmuir (m2 g–1)	V t (cm3 g–1)	V micro (cm3 g–1)	average pore size (nm)
10ENR-CNF	103.86	485.67	1.419 × 10–1	0.00397	2.733
20ENR-CNF	589.12	1589.07	6.338 × 10–1	0.0778	2.152
30ENR-CNF	689.48	1237.82	6.245 × 10–1	0.2179	1.812
50ENR-CNF	140.99	326.12	1.434 × 10–1	0.0388	2.035
70ENR-CNF	452.50	492.35	2.292 × 10–1	0.1706	2.026

^aSSA_BET, BET specific surface area.

^bSSA_Langmuir, Langmuir specific surface area.

^c V _t , total pore volume.

^d V _micro, micropore volume.

The 10ENR-CNF sample had the lowest BET surface area (103.86 m² g^–1), and with increasing amounts of ENR, the surface area increased to 452.50 m² g^–1 (70ENR-CNF), 589.12 m² g^–1 (20ENR-CNF), and 689.48 m² g^–1 (30ENR-CNF), with the surface area of 140.99 m² g^–1 (50ENR-CNF). The sharp decrease in the BET surface area of 50ENR-CNF is attributed to a morphological transition that is unique to this specific blend ratio. At the 50 wt % threshold, the blended ENR and PAN polymer likely achieves a state of maximum homogeneity, resulting in the sealing of larger micropores and mesopores that are typically accessible to N₂, thereby restricting N₂ adsorption. The Langmuir surface area was highest for the 20ENR-CNF sample at 1589.67 m² g^–1. A higher Langmuir surface area with a lower BET surface area for all of the samples suggested unfavorable interactions with the adsorbate gas, causing the Langmuir model to overestimate the surface area, while the BET model was more sensitive to multilayer adsorption effects.

3.2. CO₂ Adsorption Performance for As-Prepared and Activated nENR-CNF

3.2.1. CO₂ Adsorption Performance

Figures (a), (b), (d), and (e) show CO₂ adsorption isotherms of the as-prepared and activated nENR-CNF at 273 and 298 K and a relative pressure of 1 bar. Across all the samples, the adsorbed volume of CO₂ increased with increasing relative pressure, characteristic of physisorption-dominated processes. The isotherms typically exhibited Type-I behavior, indicative of microporous or mesoporous structures, which are favorable for gas adsorption. , The CO₂ uptake at 273 K was consistently higher than at 298 K for all the samples, confirming the exothermic nature of CO₂ physisorption, where lower temperatures enhance adsorption capacity due to stronger adsorbate–adsorbent interactions. The CO₂ uptake capacity at 298 K was lower because of the high kinetic energy of the CO₂ molecules. A bar chart (Figure (c)) compares the CO₂ uptake capacity (measured in mmol g^–1) of the as-prepared versus the activated CNF materials at 273 K. For the as-prepared CNF (brown color), the CO₂ uptake capacities were 3.53 mmol g^–1 (20ENR-CNF) > 3.02 mmol g^–1 (70ENR-CNF) > 1.84 mmol g^–1 (50ENR-CNF) > 1.39 mmol g^–1 (10ENR-CNF) > 0.07 mmol g^–1 (30ENR-CNF). This result implied that the as-prepared 20ENR-CNF and 70ENR-CNF samples had suitable porosity and interactions for capturing CO₂ molecules compared with the other samples. , After activation by CO₂ gas (shown in red), the CO₂ uptake capacities significantly increased, resulting in 4.35 mmol g^–1 (20ENR-CNF) > 3.76 mmol g^–1 (10ENR-CNF) > 3.22 mmol g^–1 (20ENR-CNF) > 2.40 mmol g^–1 (50ENR-CNF) > 2.38 mmol g^–1 (30ENR-CNF). The activated CNF samples showed significantly enhanced CO₂ adsorption compared to their as-prepared counterparts. This improvement was attributed to the increased surface area, optimized pore structure, and the chemical surface functional groups, which provided additional active sites and stronger interactions with CO₂ molecules. A high improvement after physical activation was shown by the 10ENR-CNF sample (increment of 2.37 mmol g^–1) and the 30ENR-CNF sample (increment of 2.31 mmol g^–1).

7.

CO₂ adsorption of (a) as-prepared CNF at 273 K and (b) activated CNF at 273 K. (c) A bar graph comparing CO₂ uptake capacities of at 273 K. The CO₂ adsorption of (d) as-prepared CNF at 298 K and (e) activated CNF at 273 K. (f) A bar graph comparing CO₂ uptake capacities (measured in mmol g^–1) with isosteric heat of adsorption for activated CNF at 1.0 mmol g^–1 CO₂ adsorption.

Figure (f) presents the CO₂ uptake capacity as a function of temperature (bar graph) and the isosteric heat of adsorption (Q _st) (line graph), evaluated at a CO₂ uptake of 1.0 mmol g^–1 to elucidate the adsorption behavior for CNF materials with different ENR contents (summarized in Table ). The dashed blue line with hexagonal markers represents the Q _st values, which decreased significantly from 46.36 kJ mol^–1 for 10ENR-CNF to 28.18 kJ mol^–1 for 20ENR-CNF and subsequently remained relatively stable at approximately 25–28 kJ mol^–1 for further increases in ENR content. This result suggests that the initial strong interaction between CO₂ molecules and the adsorbent surface was weakened with increasing ENR content, possibly due to variations in surface chemistry and/or pore structure of the CNF materials. The higher Q _st value of 10ENR-CNF indicates stronger CO₂-binding interactions, which may originate from enhanced interactions between CO₂ molecules and oxygen-containing functional groups. In contrast, the lower Q _st values observed for samples with higher ENR contents suggest weaker adsorbate–adsorbent interactions dominated by physisorption, concurring with previous studies. , However, all samples exhibited Q _st values below 50 kJ mol^–1, indicating that CO₂ adsorption predominantly occurred through physical adsorption mechanism. − The high Q _st value of 10ENR-CNF can be attributed to stronger surface interactions, including possible hydrogen-bonding interactions involving oxygen-containing functional groups. These findings provide insights into optimizing CNF synthesis for CO₂ capture applications, with 20ENR-CNF showing potential for further investigation due to its balanced adsorption strength and surface properties. Furthermore, the Q _st values increased gradually with increasing CO₂ uptake for all active samples, as shown in Figure S1, indicating enhanced adsorption energy at higher surface coverage. This behavior may be associated with the progressive occupation of high-energy adsorption sites and the filling of narrow pores, which strengthen the interactions between CO₂ molecules and the pore walls. Additionally, oxygen-containing functional groups may provide additional polar adsorption sites, thereby enhancing CO₂ affinity, as discussed above.

2. CO₂ Uptake Capacity of CNF and Isosteric Heat of Adsorption (Q _st) of Activated nENR-CNF.

samples	CO2 uptake capacity				Q st at 1 mmol g–1 (kJ mol–1)
	T = 273 K		T = 298 K
	(cm3 g–1)	(mmol g–1)	(cm3 g–1)	(mmol g–1)
as-prepare CNF
10ENR-CNF	31.15	1.39	21.03	0.94	not applicable
20ENR-CNF	79.12	3.53	48.55	2.17	not applicable
30ENR-CNF	1.46	0.07	0.90	0.04	not applicable
50ENR-CNF	41.13	1.84	31.51	1.41	not applicable
70ENR-CNF	67.55	3.02	50.48	2.25	not applicable
activated CNF
10ENR-CNF	84.20	3.76	29.86	1.33	47.35
20ENR-CNF	97.45	4.35	62.72	2.80	28.18
30ENR-CNF	53.39	2.38	32.66	1.46	28.21
50ENR-CNF	53.87	2.40	37.49	1.67	25.24
70ENR-CNF	72.06	3.22	52.28	2.33	27.94

Table summarizes the CO₂ uptake capacities at 273 and 298 K for various PAN-based porous carbon nanofibers compared with those of the pristine PAN@CNF sample at a pressure of 1 bar, as presented in Table . Before activation, the 20ENR-CNF sample exhibited a relatively low CO₂ uptake capacity; however, after activation, it achieved the highest adsorption performance among all of the prepared samples. Compared with those of other reported PAN-based CNFs at 298 K, the adsorption capacity of the synthesized materials was within the medium-to-high range. Nevertheless, a key advantage of this study is the reduced requirement of synthetic precursors during the fabrication of the porous carbon nanofibers. The 70ENR-CNF sample, which contained the highest ENR loading, exhibited an acceptable CO₂ uptake capacity of 2.25 mmol g^–1 (as-prepared), demonstrating the potential for simplifying the preparation process and reducing the consumption of synthetic precursors.

3. Comparison of the Recently Reported Carbon Nanofiber-Based CO₂ Adsorbent Materials with This Work.

carbon nanofiber samples	temperature/pressure (K)/(bar)	CO2 uptake (mmol g–1)	refs
PAN@CNF	298/1	2.52
PAN/poly(methyl methacrylate)	298/1	3.16
PAN/melamine ACNFs	298/1	1.44
PAN/polyvinylidene fluoride	298/1	2.21
PAN/polyvinylpyrrolidone	298/1	3.11
hollow activated carbon nanofibers	298/1	3.03
PAN/Ni/natural rubber	298/1	1.18
as-prepared 20ENR-CNF	273/1	3.53	this work
as-prepared 20ENR-CNF	298/1	2.17	this work
activated 20ENR-CNF	273/1	4.35	this work
activated 20ENR-CNF	298/1	2.80	this work

3.2.2. Correlation with Surface Area and Porosity

The relationship between the CO₂ adsorption capacity (mmol g^–1, red squares) and pore width (nm, blue triangles) is presented in Figure (a). The CO₂ adsorption capacity initially increased from 10ENR-CNF to 20ENR-CNF, reaching a maximum at 20ENR-CNF (4.35 mmol g^–1), then sharply decreased at 30ENR-CNF, and remained relatively stable for 50ENR-CNF and 70ENR-CNF. The pore width showed a decreasing trend from 10ENR-CNF (2.73 nm) to 30ENR-CNF (1.81 nm), reaching a minimum at 30ENR-CNF and then increasing at 50ENR-CNF (2.03 nm) and 70ENR-CNF (2.02 nm). This relationship suggested that a moderate pore width (not too large or too small) was favorable for CO₂ adsorption with the highest capacity observed when the pore width ranged between 2.02 and 2.73 nm.

8.

(a) CO₂ adsorption capacity versus pore width and (b) CO₂ adsorption capacity versus BET specific surface area of activated nENR-CNF samples.

The relationship between the CO₂ adsorption capacity (mmol g^–1, red squares) and BET specific surface area (m² g^–1, blue triangles) is presented in Figure (b). The CO₂ adsorption capacity trend was similar to that in Figure (a). The BET specific surface area increased sharply from 10ENR-CNF (103.86 m² g^–1) to 30ENR-CNF (689.48 m²/g) and then significantly dropped at 50ENR-CNF (140.99 m² g^–1) before increasing again at 70ENR-CNF (452.50 m² g^–1). The highest surface area of the 30ENR-CNF sample did not correspond to the highest CO₂ adsorption capacity of the 20ENR-CNF sample, indicating that the surface area was not the major factor in determining CO₂ uptake. ,, The relationship between surface area and adsorption capacity suggested that other factors than surface area and pore size, such as pore size distribution, surface chemistry, or the presence of functional groups, may play a critical role in the CO₂ adsorption performance.

3.3. Kinetic Studies

Table reports the kinetic data of CO₂ adsorption on the 20ENR-CNF sample, evaluated by matching with the experimental data from the pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD) models. PFO, modeled for gas adsorption, describes the interfacial contact between CO₂ molecules and the active sites of the adsorbent. The PSO model results suggested that gas adsorption kinetics was a rate-limiting step as a chemisorption process involving chemical interactions on the adsorbent surface. The IPD model demonstrated the speed of the CO₂ molecule adsorption via pore diffusion. The R ² value indicated the level of fit of each model to the experimental data. The PFO and PSO models provided a poor fit to the experimental data, with the PFO model showing a higher R ² value, reflecting dominant physisorption behavior. The IPD model (Figure c) consisted of two steps. The first displayed the diffusion of CO₂ from the bulk phase into the adsorbent pores, while the second was the CO₂ adsorption equilibrium on the adsorbent surface. The R ² values ranged from 0.9331 to 0.9968, suggesting that CO₂ adsorption resulted from external diffusion from bulk CO₂ and also from intraparticle diffusion within the pores. The as-prepared 20ENR-CNF sample presented the highest kinetics at 273 K. As depicted in Figure (c), the slope of the first curve (K₁) indicated slow external diffusion, while the second curve (K₂) showed faster intraparticle diffusion.

4. Adsorption Constants of Various Kinetic Models for 20ENR-CNF Samples.

model	20ENR-CNF sample
	as-prepared CNF		activated CNF
	273 K	298 K	273 K	298 K
Pseudo-first order
k 1 (min–1)	0.0229	0.0266	0.0241	0.0301
R 2	0.8653	0.8913	0.8654	0.8138
Pseudo-second order
k 1 (g mmol–1 min–1)	0.0005	0.0055	0.0006	0.0017
R 2	0.5591	0.2350	0.5551	0.3085
intraparticle diffusion
k 1 (mmol g–1 min–1)	3.8944	2.6376	2.9710	2.0211
R 2	0.9564	0.9950	0.9331	0.9953
k 2 (mmol g–1 min–1)	26.7380	15.1450	18.5710	10.1800
R 2	0.9952	0.9964	0.9954	0.9968

9.

Kinetic models for the CO₂ adsorption of the as-prepared 20ENR-CNF sample at 273 K for (a) the pseudo-first-order model, (b) the pseudo-second-order model, and (c) intraparticle diffusion model.

3.4. Pressure Swing Adsorption

Five cycles of pressure swing regeneration were carried out to investigate the recyclability of CO₂ adsorption at 273 and 298 K under 1 bar (Figure (a) and (b)). The first three cycles of recyclability gave excellent CO₂ uptake at both temperatures. After three cycles, the uptake capacities gradually decreased because some CO₂ molecules were retained in the pores of the material. For complete regeneration, this material requires high temperature and low pressure to pull out the residual CO₂ molecules as shown in Figure (c). Compared to the first cycle, the CO₂ uptake capacity negligible changes in the fifth cycle. The 20ENR-CNF sample had a high CO₂ adsorption rate with excellent recyclability. However, considering the CO₂ adsorption capacities of the 20ENR-CNF sample obtained from the temperature swing adsorption experiment, as shown in Figure S2, the CO₂ adsorption capacities are slightly lower than those obtained from the pressure swing adsorption experiment. The comparison of CO₂ and N₂ adsorption isotherm (298 K) is shown in Figure (d). Because the N₂ adsorption capacitor was extremely low compared with the CO₂ adsorption capacity over the investigated pressure range, the CO₂/N₂ selectivity was evaluated using the ratio of Henry’s Law constants rather than the Ideal Adsorbed Solution Theory (IAST). Under these conditions, IAST calculations yielded unrealistically high selectivity values owing to the high sensitivity of the prediction to the fitted N₂ adsorption isotherm. Based on the ratio of Henry’s Law constants ( $k_{\mathrm{H},\mathrm{C}{\mathrm{O}}_2}/k_{\mathrm{H},{\mathrm{N}}_2}$ ) at 298 K, the 20ENR-CNF sample exhibited a selectivity ( $S_{\mathrm{C}{\mathrm{O}}_2/{\mathrm{N}}_2}$ ) of 17.83. This value indicates a strong preferential adsorption of CO₂ over N₂, which is a critical requirement for practical postcombustion carbon capture applications.

10.

Pressure swing regeneration experiment of the 20ENR-CNF sample for five cycles of CO₂ uptake at (a) 273 K and (b) 298 K. (c) Cycling study of CO₂ adsorption after thermal regeneration and (d) CO₂/N₂ selectivity test at 298 K and 1 bar.

4. Conclusions

PAN-based porous carbon nanofibers were successfully prepared by using natural resources as an ENR polymer. The ENR was used as a copolymer, with up to 80 wt %, along with a PAN precursor, to generate porous carbon nanofibers after the carbonization process. Results showed that modifying carbon nanofiber adsorbents by creating porosity and increasing interface interaction from natural rubber and CO₂ activation enhanced CO₂ adsorption performance. The ENR content significantly impacted the surface area, CO₂ uptake capacity, and kinetic adsorption of the porous CNFs, with the CO₂ uptake capacity dependent on the surface area and also on porosity and interface interactions. The 20ENR-CNF sample achieved the highest performance for CO₂ gas molecule adsorption in both the as-prepared and activated forms. Interestingly, the 20ENR-CNF sample achieved the fastest particle adsorption through five cycles of pressure swing adsorption testing (adsorption/desorption) at low temperatures and room temperature. Our findings can be used to guide the development of advanced adsorbent materials from natural sources to improve the CO₂ adsorption applications.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c02984.

• Kinetic adsorption parameters for as-prepared and activated nENR-CNF materials; isosteric heat of adsorption for all samples; and temperature swing adsorption measurement of CO₂ adsorption capacity for the 20ENR-CNF sample (PDF)

SC: conceptualization, methodology, analysis, and writingoriginal draft. MS: conceptualization, methodology, analysis, writingreview and editing, funding acquisition, and resources.

The authors declare no competing financial interest.