Carbon-Nanotubes- and Porous Organic Polymers-Based Porous Fluids for CO₂ Capture ^†

Abstract

Emissions of carbon dioxide are considered to be the major factors leading to climate change. Current technologies for CO₂ capture include chemical and physical absorption, membrane separation, and cryogenic distillation. Solid adsorbents are highly effective, and emerging porous liquids—solid adsorbents dispersed in a compatible liquid—represent a promising alternative for CO₂ capture. In this study, commercial and synthetic porous carbon nanomaterials were dispersed in a 1 wt.% aqueous solution of sodium dodecyl sulfate and compared on the efficiency of CO₂ uptake. The experimental results confirmed that CO₂ uptake is enhanced in porous liquids employing surface-modified carbon nanotubes (69 mmol/L) and covalent triazine frameworks, with triazine-based nanomaterials exhibiting superior CO₂ uptake performance (76 and 82 mmol/L) due to their increased polar group number.

1. Introduction

The rising levels of atmospheric carbon dioxide [1], the major greenhouse gas, require the development of effective and scalable CO₂ capture systems. A highly efficient solution is needed for CO₂ capture from CO₂-containing gas streams, such as natural gas, flue gas, etc. The widely used technologies for CO₂ dissolution and separation include amine absorption [2], zeolite adsorption [3], and cryogenic distillation [4]. Nevertheless, these methods suffer from substantial drawbacks. Challenges associated with amine absorption include corrosion of carbon steel, high energy demand for solvent regeneration, and solvent processing issues such as the selection of suitable amines, optimization of concentration, and control of viscosity. These limitations have been widely discussed in the literature [2,5]. Zeolite adsorption also requires high energy for zeolite regeneration, including high temperature or low pressure. Additionally, purified gases often include moisture, which adsorbs on hydrophilic zeolites, occupying active adsorption sites and significantly reducing the effective CO₂ uptake under humid conditions [6]. Cryogenic distillation of CO₂ is effective at high CO₂ concentrations, but this method requires low temperatures, resulting in high energy consumption and operational costs [7]. The cryogenic separation of CO₂-containing gas mixtures demonstrates a high efficiency at high CO₂ concentrations in carbon-enriched gas mixtures, but limited applicability in gases with low CO₂ concentrations. Furthermore, equipment blockage can occur due to the formation of dry ice under cryogenic operating conditions [8].

Porous liquids, consisting of porous solid CO₂ sorbents dispersed in liquid media, represent an alternative method of separating CO₂-containing gas streams. The concept of porous liquids was proposed by James et al. in 2007 [9]. Depending on their composition, porous liquids can be categorized into three types: “near-porous” liquid hosts that form true single-phase solutions without a distinct boundary between the porous solid and the solvent (Type I), liquids containing solid porous molecular cages dissolved in sterically hindered solvents (Type II), and porous molecules dispersed in solvents (Type III) [9].

Examples of CO₂ uptake in porous liquids in the literature (

Table A1. Porous liquids and their CO₂ uptake performance.

Porous Filler (Loading, wt.%)	Liquid Media	Porous Liquid Type	CO2 Uptake, mmol/g	Testing Conditions	Ref.
UiO-66(185)@xPDMS (37 wt.%)	PDMS	III	1.40	298 K, 1 bar	[35]
UiO-66-NH2@xPDMS (33.3 wt.%)	PDMS	III	1.30	298 K, 1 bar
UiO-66-Br2@xPDMS (36 wt.%)	PDMS	III	0.60	298 K, 1 bar
Zeolite Rho (12.5 wt.%)	Genosorb 1753	III	0.30	298 K, 1 bar	[36]
Zeolite Rho (25 wt.%)	Genosorb 1753	III	0.70	298 K, 1 bar
Zeolite Rho (12.5 wt.%)	Genosorb 1753	III	1.10	298 K, 5 bar
Zeolite Rho (25 wt.%)	Genosorb 1753	III	1.60	298 K, 5 bar
15-C-5-PL	Anionic covalent cage	I	0.40	298 K, 10 bar	[10]
18-C-6-PL	Anionic covalent cage	I	0.40	298 K, 10 bar
CP-SIT (5 wt.%)	M2070	III	1.50	298 K, 25 bar	[37]
F108-SIT (5 wt.%)	M2070	III	2.00	298 K, 25 bar
F127-SIT (5 wt.%)	M2070	III	2.20	298 K, 25 bar
UiO-66 (1 wt.%)	M2070	III	0.89	298 K, 30 bar	[11]
UiO-66 (1 wt.%)	M2070	III	0.20	298 K, 10 bar
UiO-66	PEGS		0.60	298 K, 10 bar	[12]
ZIF-8 (5 wt%)	[P6,6,6,14][NTf2]	III	0.34	303 K, 5 bar	[13]
ZIF-8 (5 wt%)	[P6,6,6,14][NTf2]	III	0.50	303 K, 5 bar
ZIF-8	[Bpy][NTf2]	III	0.10	298 K, 1 bar	[14]
CNT-P (0.004 wt.%)	1 wt.% SDS	III	0.06	298 K, 1 bar	This work
CNT-M (0.004 wt.%)	1 wt.% SDS	III	0.07	298 K, 1 bar
CTF-DIPEA (0.004 wt.%)	1 wt.% SDS	III	0.08	298 K, 1 bar
CTF-K2CO3 (0.004 wt.%)	1 wt.% SDS	III	0.08	298 K, 1 bar

) indicate a wide range of performance depending on the dispersed nanomaterial and liquid medium. Porous liquids based on porous organic cages demonstrate an enhancement of CO₂ dissolution compared to neat solvents, but the overall pore concentration is limited due to low cage loading [10]. Type III porous liquids based on MOF particles demonstrate significantly higher CO₂ uptake performances at ambient conditions compared to the pure liquid phase [11,12,13]. For example, Type II and III porous liquids based on ZIF-8 dispersed in ionic liquids exhibit enhanced CO₂ uptake relative to the neat ionic liquid, suggesting the contribution of solid porosity within the fluid medium [13,14]. Although absolute values vary with system design, general trends show that tailored porous liquid systems can achieve CO₂ uptake capacities that rival or exceed early porous cage liquids, indicating significant performance potential as both sorbents and fluid materials in gas separation applications. These approaches have expanded the range of nanoporous materials beyond classic physical absorbents and point toward porous liquids as a new class of materials for CO₂ capture.

Owing to their tunable pore environment, low viscosity, and enhanced gas dissolution, porous liquids show potential for selective and reversible CO₂ capture. Porous liquids show promise for CO₂ capture, improving its dissolution in liquid media by combining sorbent porosity and selectivity and the fluidity of a medium [15]. Additionally, the porous filler is uniformly distributed in liquid matrices without interfacial defects [16]. Furthermore, liquids usually do not suffer from mechanical fatigue, physical aging, or plasticization, which are commonly observed in solid sorbents [15]. There is no need to reconstruct existing industrial units (e.g., amine absorption setups), because porous liquids can be embedded in existing units. Their fluid nature provides potential regeneration under milder conditions.

Despite these advantages, the design of porous liquids remains challenging. Achieving high gas uptake requires maintaining open, unoccupied pores while simultaneously ensuring compatibility and stability of the dispersed components within the host liquid. Mass transfer between liquid and gas may be slower than in conventional CO₂ capturing systems, particularly at low gas pressures [17]. The viscosity and diffusivity of the liquid medium can influence uptake kinetics, and optimization of nanomaterial loading is necessary to balance performance and cost [18].

Porous liquids might exhibit superior performance in scenarios where conventional solid adsorbents are difficult to process, for example in flowing systems, microreactors, or packed columns requiring fluidity, or when selective CO₂ capture under mild conditions is desirable [15].

In this study, commercial and synthetic porous carbon nanomaterials and frameworks were obtained. Nanomaterials such as carbon nanotubes (CNT) and covalent triazine frameworks (CTF) were dispersed in an aqueous solution and evaluated according to their efficiency of CO₂ uptake. CO₂ capture performance of carbon nanotubes and covalent triazine frameworks has been extensively studied in many research works, demonstrating exceptional values of CO₂ uptake [19,20,21,22,23,24]. However, porous liquids containing these nanomaterials have not been studied yet. The aim of this research was to investigate the behavior of CNTs and CTFs as fillers in porous liquids and evaluate their potential to enhance CO₂ dissolution. The present work advances the porous liquid concept in a simple and scalable direction by employing an aqueous continuous phase (1 wt.% SDS aqueous solution) and dispersing nanoporous solids (CNTs) and, for the first time, covalent triazine frameworks to generate Type III porous liquid formulations under ambient conditions. Key distinguishing aspects relative to prior porous liquid reports include: (i) the use of a low-cost, low-toxicity aqueous/surfactant medium rather than neat ionic liquids or molecular solvents; (ii) the direct comparison of pristine versus surface-modified carbon nanotubes (CNT-P vs. CNT-M) and two CTF synthesis routes (CTF-DIPEA vs. CTF-K₂CO₃) within the same formulation strategy; and (iii) an emphasis on CO₂ uptake in low-pressure conditions (298 K, 1 bar) using a manometric method, enabling rapid screening of formulation performance. In the described workflow, the best-performing formulation (CTF-DIPEA in 1 wt.% SDS) achieves the highest CO₂ uptake among the tested liquids, indicating that CO₂ affinity (e.g., basic nitrogen sites and possible reversible chemisorption) can outweigh purely geometric porosity metrics, highlighting the importance of coupling surface chemistry with a stable dispersion strategy.

2. Materials and Methods

1,4-Dioxane, cyanuric chloride, potassium carbonate, nitric acid (65%), sulfuric acid (98%), and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich (St. Louis, MO, USA) at purities >99%. 1,4-Phenylenediamine (>98%) and N,N-diisopropylethylamine (DIPEA, >99%) were obtained from ABCR (Karlsruhe, Germany). Acetone and isopropanol (reagent grade) were provided by Rushim (Moscow, Russia). Single-walled carbon nanotubes (SWCNTs) were supplied by Universal Additives LLC (Novosibirsk, Russia). All chemicals were used as received without further purification.

2.1. Synthesis of Nanomaterials

2.1.1. Synthesis of CTF-DIPEA

Synthesis of CTF-DIPEA was performed according to a reported procedure [25]. The synthesis route of CTF-DIPEA is shown in Figure 1. A mixture of 1,4-phenylenediamine (10.2 mmol) in 1,4-dioxane (30 mL) was added dropwise over 1 h to a mixture of cyanuric chloride (13.6 mmol) and N,N-diisopropylethylamine (9 mL) under an argon atmosphere. The mixing was performed with vigorous stirring at 15 °C, resulting in a light brown mixture. The reaction temperature was raised to 25 °C and maintained for 2 h, then further increased to 65 °C and held for 21 h under reflux. After cooling, the resulting solid was ground in a mortar. The product was repeatedly washed with isopropanol via centrifugation and subsequently left to soak in isopropanol for 16 h, followed by additional washes with fresh isopropanol. The solvent was removed using a rotary evaporator. Finally, the CTF-DIPEA powder was dried in a drying cabinet at 120 °C and 10 mbar overnight, yielding 83 wt.% of light brown solid.

2.1.2. Synthesis of CTF-K₂CO₃

CTF-K₂CO₃ was synthesized via a modified procedure [26] with a precise stoichiometry (Figure 1). A solution of 1,4-phenylenediamine (15 mmol) in anhydrous 1,4-dioxane (50 mL) was mixed with K₂CO₃ (30 mmol) and cooled to 15 °C under an argon atmosphere, followed by the dropwise addition of a solution of cyanuric chloride (10 mmol) in 1,4-dioxane over 2 h. The reaction mixture was heated to room temperature over 1 h and then refluxed at 90 °C for 72 h. The resulting precipitate was collected by filtration and washed sequentially with water, isopropanol, and acetone. Drying at 120 °C and 10 mbar in a drying cabinet overnight yielded 84 wt.% of light pink solid.

2.1.3. Surface Modification of Carbon Nanotubes

Pristine SWCNTs (CNT-P) (0.8 g) were mixed with a 100 mL mixture of HNO₃/H₂SO₄ (v/v = 1:3) and refluxed for 4 h at 70 °C, maintaining regular stirring. After cooling to room temperature, the mixture was diluted with deionized water, followed by filtering through a 0.2 µm Millipore mixed cellulose ester membrane until the pH of the filtrate reached 7. Then filtered oxidated SWCNTs were dried in a drying cabinet at 80 °C and 10 mbar overnight, yielding 73 wt.% of modified CNTs, named as CNT-M (Figure 2).

2.2. Formulation of Porous Liquids

A 1 wt.% solution of SDS was prepared in deionized water under stirring until complete dissolution. To ensure micelle formation, the 1 wt.% (34 mM) SDS concentration was chosen well above the critical micelle concentration (CMC) of SDS, equal to ~8 mM at room temperature [18,27,28]. This solution was subsequently used in the preparation of porous liquids with nanomaterials. For blank measurements, deionized water and the 1 wt.% SDS solution were utilized.

Four separate samples of porous liquids were prepared by adding 2 mg of different nanomaterials (0.004 wt.%) to 50 mL of 1 wt.% SDS, followed by 30 min of sonication (300 W, 20 kHz) (Figure 3). To prevent overheating, the flask containing the solution was maintained in an ice bath, and sonication was performed in either continuous mode or with alternating 5 min pulses and 5–10 min pauses. The loading of nanomaterials was chosen as the starting point to observe the presence of statistically important differences in CO₂ uptake between 1 wt.% SDS and the liquid systems loaded with porous nanomaterials. This content of porous materials was probed to avoid the disruption of SDS micelle stability as well as the stability of the entire system.

2.3. Characterization Techniques

The FT-IR spectra were recorded from 4000 to 500 cm⁻¹ on a Thermo Scientific Nicolet (Waltham, MA, USA) iS50 FT-IR spectrometer. CNTs were analyzed in KBr pellets, whereas CTF-DIPEA and CTF-K₂CO₃ were characterized by attenuated total reflection.

Thermogravimetric analysis was performed using the Discovery TGA 5500 system from TA Instruments (New Castle, DE, USA) with a heating rate of 10 °C/min from 26 to 600 °C using a nitrogen flow of 15 mL/min and an initial sample weight of ~2–4 mg.

Brunauer–Emmett–Teller (BET) surface area and pore volume analysis was conducted by measurements of nitrogen adsorption isotherms at 77 K using a BELSORP Mini-X device (MicrotracBEL, Osaka, Japan). Preliminary preparation of the samples was carried out by vacuuming to 2–4 Pa at a temperature of 120–350 °C (depending on the sample type) for 12 h. The total specific surface area and the total pore volume were measured via the BET method at a relative partial pressure of p/p₀ = 0.3; mesopore size distribution, mesopore area and volume were determined using the Barrett–Joyner–Halenda method; micropore size distribution was estimated using the adsorption curve.

XRD patterns were obtained using a Thermo Scientific (Waltham, MA, USA) ARL EQUINOX 1000 X-ray Diffractometer to determine the materials’ crystallographic structures. CuKα radiation with a wavelength of λ = 1.54 Å was used over a diffraction angle of 2θ ranging from 10° to 90°.

XPS analysis was performed on a PREVAC EA15 (Prevac, Poland) spectrometer equipped with a high-resolution hemispherical analyzer. Characteristic non-monochromatic Al Kα X-ray radiation (1486.6 eV, 150 W) was used to excite the spectra. The residual gas pressure during measurements was 5 × 10⁻⁹ mbar. The binding energy scale was pre-calibrated using the photoelectron lines of the core levels of gold (Au4f_7/2—84.0 eV) and silver (Ag3d_5/2—368.3 eV). Powdered samples were applied to double-sided conductive adhesive tape and mounted on a standard holder. Each survey and high-resolution spectrum was fitted with constrained Gaussian components on a linear background.

2.4. Experimental Determination of CO₂ Uptake in Liquids

Nanomaterials were compared for their efficiency as the porous fillers in fluids. Efficiency of porous fluids was estimated and compared with blank measurements for water and an aqueous solution of surfactant. The CO₂ uptake of the liquid samples was determined volumetrically at 298 K and 101.3 kPa, following a modified procedure from the literature [29].

A predetermined mass of dry ice (0.5–1.0 g) was weighed using an analytical balance and immediately transferred into a sealed 500 mL flask containing 20–25 mL of the analyte liquid. The flask, equipped with a calibrated manometer (working range 0–1.6 bar), was maintained under vigorous stirring to facilitate the complete evaporation of CO₂. The pressure of evaporated CO₂ was registered by the manometer, previously calibrated via the evaporation of dry ice in deionized water (50 mL). The pressure measurement for each sample was performed three times, and the average value was calculated. The detailed calculation methodology is described below.

Volume of CO₂ released from dry ice (mL):

$$V_{{CO}_2}={\displaystyle \frac{m_{{CO}_2}}{M_{{CO}_2}}}·V_M,$$

(1)

where

• $m_{{CO}_2}$—mass of dry ice (g);
• $M_{{CO}_2}$—molar mass of CO₂ (44 g/mol);
• $V_M$—molar volume of CO₂ (22,400 mL/mol).

Overpressure generated by dry ice (bar):

$$P_{theor}={\displaystyle \frac{V_{{CO}_2}}{V_{headspace}}},$$

(2)

where

• $V_{headspace}$—volume of headspace (mL), calculated as volume of flask (500 mL)—volume of liquid (mL).

Correlation pressure was calculated from manometer uncertainty (bar):

$$P_{corr}=P_{meas}·c$$

(3)

where

• $P_{meas}$—pressure registered by manometer (bar);
• $c$—correlation coefficient (1.1271) derived from calibration against measured pressure upon dry ice evaporation in deionized water and theoretical pressure calculated by the Mendeleev–Clapeyron equation (Figure 4).

Pressure difference (bar):

$$dP=P_{theor}-P_{corr}$$

(4)

Volumetric CO₂ uptake (mmol/L):

$${CO}_{2}uptake={\displaystyle \frac{\frac{dP}{P_{theor}}·\frac{m_{{CO}_2}}{M_{{CO}_2}}}{V_s}}·{10}^6$$

(5)

The dry ice evaporation method can cause experimental uncertainties, including non-uniform sublimation of dry ice and dissolution of CO₂, potential gas leakage, equilibrium assumptions, and non-isothermal fluctuations. This method is extensively described in the work of Truong et al. [29], which served as the experimental basis for the present study. The method implicitly assumes constant temperature and ideal gas behavior; deviations from these assumptions introduce uncertainties in the calculated CO₂ uptake. Non-isothermal fluctuations lead to changes in gas expansion and compression, which influence the measured CO₂ volume and pressure inside the flask. Specifically, a higher ambient temperature causes thermal expansion of CO₂, potentially resulting in an overestimation of the available CO₂ amount, while a decrease in temperature leads to gas compression and may cause underestimation. Additionally, temperature gradients can alter the sublimation kinetics of dry ice, affecting the CO₂ generation rate and the time required to reach adsorption equilibrium.

However, the method is enough for reliable qualitative and quantitative CO₂ adsorption measurements. According to Figure 4, the measured CO₂ pressure closely coincides with the theoretical pressure calculated using the ideal gas law, yielding a high correlation coefficient (R² = 0.9959). This agreement indicates that the behavior of CO₂ generated from dry ice in the experimental apparatus can be well approximated by ideal gas behavior under the applied conditions.

3. Results and Discussion

3.1. Nanomaterials Characterization

3.1.1. FT-IR Spectroscopy

The FT-IR spectra of pristine and modified CNTs are shown in Figure 5a. The most prominent feature in both spectra is a peak at 3425 cm⁻¹, indicating the presence of hydroxyl (–OH) groups on the CNTs [30]. A distinctive absorption band at 1660–1630 cm⁻¹ occurred due to the stretching vibrations of carbon–carbon (–C=C) and carbon–oxygen (–C=O) bonds. The presence of C-H bond stretching vibrations is indicated by peaks at 2950 cm⁻¹. Signals at 1100 and 1260 cm⁻¹ are attributed to stretching vibrations of the carbon–oxygen (–C–O) bond in CNT-carboxyl functional groups resulting from the modification process.

Successful synthesis of CTFs was confirmed by FT-IR analysis (Figure 5b). Peaks at 799, 1220, 1382, 1494, and 1544 cm⁻¹ are attributed to –C=N stretching vibrations and indicate the presence of benzene and triazine structures. The signal observed at 982 cm⁻¹ is assigned to the =C–H bending vibration. The more intense absorption band observed at 3270–3400 cm⁻¹ for CTF-K₂CO₃ is consistent with a higher degree of cross-linking, resulting from the use of stoichiometric reagent ratios.

3.1.2. Thermogravimetric Analysis

Thermogravimetric analysis revealed distinct thermal stability profiles between the materials (Figure 6). CNT-P exhibited the highest stability, retaining a residual mass of 93 wt.% at 600 °C. Chemical modification of CNTs resulted in decreased thermal stability, with a final mass of 64 wt.% at the same temperature. CTF-DIPEA underwent negligible mass loss up to 150 °C and rapid decomposition, resulting in the lowest residual mass of 47 wt.% at 600 °C among the synthesized nanomaterials. In contrast, CTF-K₂CO₃ demonstrated superior thermal stability compared to CTF-DIPEA, retaining 71 wt.% at 600 °C.

3.1.3. BET Surface Area and Pore Structure Analysis

Nitrogen adsorption–desorption isotherms measured at 77 K indicate that all studied nanomaterials exhibit type IV isotherms, which is characteristic of mesoporous materials (Figure 7). CNT-P display a type IV isotherm with an H4-type hysteresis loop (Figure 7a), typically associated with slit-like pores originating from aggregates or structural defects. Among the investigated materials, CNT-P possess the highest specific surface area (460.6 m²/g) and total pore volume (1.03 cm³/g), confirming their well-developed porous network. CNT-M, which have the second-highest BET surface area (64.6 m²/g) and total pore volume (0.25 cm³/g), exhibit an H1-type hysteresis loop (Figure 7b). This loop shape is indicative of cylindrical mesopores and suggests that the acid treatment of CNTs induced oxidation of the carbon nanotube framework, leading to more open-ended CNTs. CTF-DIPEA reveals a type IV isotherm with a mixed H1/H2 hysteresis loop (Figure 7c). The BET surface area (3.4 m²/g) and total pore volume (0.02 cm³/g) are the lowest among the samples, implying a significantly reduced overall porosity. Nevertheless, the mixed hysteresis loop suggests the coexistence of both uniform mesopores and more complex pore structures, pointing to a relatively homogeneous but less developed porous system. Finally, CTF-K₂CO₃ presents intermediate values of BET surface area (55.3 m²/g) and total pore volume (0.21 cm³/g) and features an H1-type hysteresis loop (Figure 7d), indicating the presence of relatively uniform mesopores characteristic of partially ordered porous structures. The values of BET surface area and total pore volume of the synthesized nanomaterials are shown in

Table 1. BET surface area and total pore volume of the nanomaterials.

Nanomaterial	BET Surface Area, m2/g	Total Pore Volume, cm3/g
CNT-P	460.6	1.03
CNT-M	64.6	0.25
CTF-DIPEA	3.4	0.02
CTF-K2CO3	55.3	0.21

.

Mesopore size distributions were evaluated via the BJH method (desorption branch), while micropore presence was assessed using the low-pressure region of the N₂ isotherm and t-plot outputs where available. Notably, CNT-P show measurable microporosity, with a t-plot micropore volume of 0.0575 cm³/g, whereas acid-modified CNT-M exhibit a much lower micropore volume (~0.00068 cm₃/g), consistent with oxidation-induced loss or blockage of micropores. The CTF samples show negligible accessible microporosity: CTF-K₂CO₃ is predominantly mesoporous (modal pore width ~22 nm) and CTF-DIPEA exhibits very low overall CO₂ uptake, suggesting either limited accessible porosity or pore blocking/collapse.

3.1.4. X-Ray Diffraction

Figure 8 demonstrates XRD patterns of CNT-P and CNT-M. CNT-P exhibit a sharp and intense diffraction peak at 2θ = 25.6°, which corresponds to the (002) reflection of graphitic carbon. The less intense peak observed at 2θ = 42.7° is associated with the (100)/(101) planes of the hexagonal carbon structure. In contrast, CNT-M display a broadening and decrease in intensity of the (002) peak, indicating a partial loss of crystallinity as a result of the oxidation process, which results in more structural defects. The disturbance of the graphitic lattice is confirmed by the peak at ~43°, which becomes broader and less intensive. XRD of CTF-DIPEA and CTF-K₂CO₃ indicated the amorphous domain, which is associated with structural defects and discontinuities within the layers. Partial crystallinity was observed at 2θ = 38°, 44° and 77° for both CTF-DIPEA and CTF-K₂CO₃, but CTF-K₂CO₃ demonstrated a higher crystallinity.

3.1.5. XPS Analysis

A consistent fitting strategy was applied to both CNT-P and CNT-M. The C 1s region was deconvoluted into graphitic sp² carbon, a defect/sp³/C–O component, a carbonyl/carboxyl component, and a π–π* stacking. The O 1s region comprised carbonyl/carboxyl oxygen, C–O/C–OH species, and a minor high-binding-energy component. Binding energies (BEs) were charge-corrected by referencing graphitic C 1s to 284.8 eV. Surface atomic percentages were derived from fitted peak areas using relative sensitivity factors (RSFs)—C 1s = 1.00, O 1s = 2.93—and normalized to (C + O). The degree of oxidation was calculated as DO_x = 100 × (O/C)_XPS.

For the CTFs the aromatic C 1s peak (284.8 eV) was used for charge correction. C 1s was fitted with phenylene C–C/C=C, a mixed C(sp²)–N/C(sp²)–OH component, and higher BE triazine-related carbon. N 1s included triazine nitrogen and a broader amine/protonated-N envelope. O 1s comprised triazinyl-O, C(sp²)–OH/adsorbed oxygen, and physisorbed H₂O/OH species, while Cl 2p was fitted as a spin–orbit doublet. Surface atomic percentages were calculated using RSFs (C 1s = 1.00, N 1s = 1.80, O 1s = 2.93, Cl 2p = 2.285). Because O 1s includes both framework and adsorbed species, the degree of polymerization is reported as an apparent surface value, estimated from residual Cl content and corrected by excluding both C–Cl and hydrolyzed triazinyl-O contributions.

The CNT-P surface remains predominantly graphitic (Figure 9a and Figure A3). The C 1s fit is dominated by sp² carbon, while O 1s shows only a small amount of oxygen functionality. The oxygen signal is attributed to pre-existing edge or oxygen defects rather than extensive oxidation. Modified CNTs (CNT-M) clearly increase the abundance of oxygen-bearing surface groups (Figure 9b and Figure A4). The oxidized sample shows strong growth in the number of C–O/C–OH groups and the carbonyl/carboxyl contribution in C 1s and O 1s, along with a relative decrease in the graphitic contribution. The oxidation metrics derived from the fitted total C 1s and O 1s signals are as follows: CNT-P exhibit C = 95.74 at.% and O = 4.26 at.% (DOx = 4.45%), whereas CNT-M show C = 82.09 at.% and O = 17.91 at.% (DOx = 21.82%).

CTF-DIPEA shows a nitrogen-rich triazine/arylamine framework together with a measurable residual chlorine signal and a stronger signal of two types of oxygen (Figure 9c and Figure A5). Because this sample was prepared with an excess of cyanuric chloride, the broad high-energy N 1s fit consists of terminal amino groups. The dominant fit with higher energy is assigned to bridge-amine/amino/protonated nitrogen atoms. The O 1s/C 1s pair also supports the presence of hydrolyzed triazine-derived C(sp²)-OH or tautomeric triazinone-like motifs. CTF-K₂CO₃ remains clearly triazine-based but shows lower surface content of Cl and O than CTF-DIPEA. CTF-K₂CO₃ shows the higher polymerization degree, consistent with higher substitution and lower residual chlorinated/hydrolyzed species at the surface (Figure 9d and Figure A6). The hydrolysis-corrected polymerization values reported here represent surface-sensitive lower-bound estimates rather than absolute bulk degrees of polymerization. CTF-DIPEA exhibits a substitution conversion of 87.99% and a hydrolysis-corrected apparent surface polymerization of 47.85%, while CTF-K₂CO₃ shows a substitution conversion of 91.22% and a corresponding value of 56.85%.

3.2. Analysis of CO₂ Uptake Performance in Liquids

The measured CO₂ uptake values for all liquid systems are summarized in

Table 2. CO₂ uptake values of liquids.

Liquid Media	Nanomaterial	CO2 Uptake of Liquid System, mmol/L 1
Deionized water	―	19
1 wt.% SDS	―	38
1 wt.% SDS	CNT-P	64
1 wt.% SDS	CNT-M	69
1 wt.% SDS	CTF-DIPEA	82
1 wt.% SDS	CTF-K2CO3	76

¹ Measured at 298 K, 101.3 kPa.

. Initial experiments were conducted using DI water and 1 wt.% SDS aqueous solution. The CO₂ uptake of DI water was determined to be 19 mmol/L. The presence of SDS led to an increase in CO₂ solubility, which can be attributed to the availability of additional active sites within SDS micelles capable of interacting with CO₂ molecules. As expected, the incorporation of surfactants significantly enhanced CO₂ dissolution, resulting in a twofold increase in uptake (38 mmol/L) compared with pure water. Further improvement in CO₂ capture was achieved by dispersing CNT-P in the water–surfactant mixture, indicating a synergistic effect between CNT-P and the micellar system. Subsequent modification of CNTs produced an additional increase in CO₂ solubility. Among the tested additives, covalent triazine frameworks demonstrated the most pronounced enhancement, yielding a CO₂ uptake approximately twice that of the water–surfactant solution. A comparison of the CO₂ uptake performance of the obtained porous liquids with literature-reported systems is provided in Table A1.

Despite exhibiting the lowest BET surface area (3.4 m²/g) and total pore volume (0.02 cm³/g) among the tested materials, CTF-DIPEA showed the greatest CO₂ uptake (82 mmol/L) when dispersed in SDS. This result suggests that chemisorption is the dominant mechanism governing the interaction between CO₂ and CTF-DIPEA in the liquid phase. The enhanced CO₂ uptake is the result of the presence of unreacted –NH– and –NH₂ functional groups in CTF-DIPEA, arising from the non-stoichiometric conditions used during its synthesis. This assumption is supported by the FT-IR spectrum, which shows weak –C=N vibrational bands indicative of incomplete linkage formation. The resulting more open structure, compared with CTF-K₂CO₃, allows –NH and –NH₂ groups to interact more effectively with dissolved CO₂, forming carbamate groups.

To confirm the formation of carbamate functionalities, the dry powder of CTF-DIPEA was saturated with CO₂, and its FT-IR spectrum was compared with the spectrum of the original CTF-DIPEA material. The procedure of CO₂ saturation was conducted using a previously reported method [31]. A CTF-DIPEA sample powder was dried overnight in a lyophilization unit. The sample was then placed in a sealed flask under a CO₂ atmosphere, evacuated, and maintained under CO₂ for 3 days to ensure complete saturation. FT-IR measurements were performed immediately after removal of sample from the flask to minimize any potential loss of adsorbed CO₂. Figure 10 shows the FT-IR spectra of the original sample (CTF-DIPEA) and CO₂-saturated sample (CTF-DIPEA-CO₂).

It was observed that CTF-DIPEA-CO₂ demonstrated a very broad O–H stretching band in the region of 2550–3300 cm⁻¹. This peak overlaps with N–H stretching vibrations, typically appearing at 3300–3500 cm⁻¹, which are weak or hidden by the O–H stretching vibrations in the spectrum of CTF-DIPEA-CO₂ and weakly visible in that of CTF-DIPEA. The N–H vibrational band becomes more pronounced due to the molecular interaction between the COOH and N-functional groups. Carbamic acids are known to form strong intramolecular hydrogen bonds, similar to carboxylic acids, and additional interaction with the triazine moiety, a strong hydrogen-bond acceptor, contributes to the observed broadening of the O–H/N–H stretching region [32].

FT-IR analysis of CO₂ chemisorption on triazine materials defined by spectral changes demonstrates shifts or new bands for the triazine ring—1350–1550 cm⁻¹ [17]—and asymmetric stretching at 2350–2500 cm⁻¹ [15], indicating interaction of CO₂ with nitrogen sites, thus forming species like carbamates or carbonates. However, XPS analysis is required to draw a complete conclusion regarding the nitrogen content and chemical states. A more detailed investigation of these aspects will be the subject of future work.

Another notable difference between the spectra of CTF-DIPEA and CTF-DIPEA-CO₂ is the presence of a carbonyl stretching band at 1680–1740 cm⁻¹, which is not characteristic of CTF-DIPEA. More intense N–H bending at 1550 cm⁻¹ supports the presence of N–H groups and indicates charge transfer interactions. Additionally, an asymmetric O–C–O stretching vibration is observed in the region 1210–1260 cm⁻¹.

To strengthen the mechanistic links between pore structure and CO₂ capture performance, especially for materials where micropores may be present but not well quantified by BJH analysis, the standard practice is to complement BET and BJH analysis (Figure A1) with t-plots (Figure A2) that identify the linear “non-microporous” region and use them to separate external surface adsorption from micropore filling.

Finally, it is important to note that for porous liquids CO₂ uptake is not governed only by the solid micropore fraction. In Type III formulations, the accessible porosity in the dispersed phase must be preserved in the liquid environment, and the continuous phase can contribute significantly via dissolution and specific CO₂ interactions. Consistent with this, the best-performing formulation (CTF-DIPEA) achieved the highest CO₂ uptake in the liquid phase despite the CTF-DIPEA solid exhibiting a very low BET surface area, suggesting the dominant roles of chemical affinity (basic nitrogen sites, and potentially reversible chemisorption/hydrogen-bonding interactions) and dispersion microstructure rather than geometric surface area alone.

4. Conclusions

In this work, we developed and evaluated new formulations of porous liquids based on pristine and surface-modified carbon nanotubes and covalent triazine frameworks dispersed in 1 wt.% SDS. Formulated liquids demonstrated enhanced CO₂ uptake compared to the pure SDS, enhancing CO₂ solubility in liquid and confirming the higher efficiency of triazine-based nanomaterials compared to nanomaterials without nitrogen functionalities.

Modification of carbon nanomaterials led to enhancement of CO₂ adsorption capacity. Porous liquids containing adsorbents with a greater number of polar groups demonstrated higher CO₂ uptake. Particularly, covalent triazine frameworks were more efficient, while surface-modified carbon nanotubes demonstrated less CO₂ uptake. Despite its low BET surface area, CTF-DIPEA, which was synthesized from cyanuric chloride and p-phenylenediamine with N,N-diisopropylethylamine and further dispersed in 1 wt.% SDS, showed the highest CO₂ uptake. The FT-IR spectrum of the CO₂-saturated powder of this sample supported the formation of carbamate species and revealed structural changes associated with CO₂ binding.

Overall, the present study underscores the potential of porous liquid formulation in the enhancement of CO₂ uptake due to chemical functionalization. Our findings contribute to the design of porous liquids with improved performance for CO₂ capture from CO₂-containing gas streams.

5. Patents

The materials presented in this work and their proposed applications are covered by IP WO2025221764A1 [33] and WO2025105974A1 [34].