Stable Reusability of Nanocellulose Aerogels with Amino Group Modification in Adsorption/Desorption Cycles for CO₂ Capture

Abstract

This study evaluated the stability and reusability of amino-functionalized nanocellulose aerogels as CO₂-adsorbent materials. The modified aerogels, synthesized via a controlled silylation using N-[3-(trimethoxysilyl) propyl] ethylenediamine (DAMO), demonstrated excellent thermal stability up to 250 °C (TGA) and efficient CO₂ adsorption through chemisorption, which was the main adsorption mechanism. The performance of the aerogels was assessed using both adsorption isotherms and the decay pressure technique, revealing that CO₂ adsorption capacity increased with higher amino group loading (4.62, 9.24, and 13.87 mmol of DAMO). At 298 K and 4 bar, CO₂ adsorption capacity increased proportionally with the amino group concentration, reaching values of 3.17, 5.98, and 7.86 mmol of CO₂ g⁻¹ polymer, respectively. Furthermore, over 20 adsorption/desorption cycles, the aerogels maintained 95% CO₂ desorption at ambient temperature, indicating their potential for industrial use. These findings highlight the aerogels suitability as stable, reusable materials for large scale CO₂ capture and storage technologies.

1. Introduction

CO₂ is considered the most important greenhouse gas, due to the continuous increase in atmospheric CO₂ concentration from 310 ppm in 1960 to 410 ppm in 2019 [1]. To reduce CO₂ concentrations, it is necessary to implement measures such as: reducing deforestation, promoting the use of more efficient energy, using renewable energy sources, and applying carbon capture and storage (CCS) as well as capture and storage utilization (CCU) technologies [2]. The main carbon capture technologies are based on absorption and adsorption operations, membrane separation, and cryogenic distillation technologies, the first two being the most studied [3].

Adsorption technologies using solid porous materials seems to be an efficient alternative for CO₂ capture due to its reusable nature, low cost, versatility, and easy operations. The literature reports on different adsorbent materials with high selectivity, including amine-modified silica, amines supported on porous carbon, and/or on other metal oxides such as alumina, zeolites, metal oxides, and metal–organic frameworks (MOFs) [4,5]. The development of some of these materials is limited due to drawbacks such as their hydrophilic nature, which requires a drying step prior to use. Therefore, the introduction of hydrophobic sites has been considered to overcome this limitation. In fact, hydrophobic microporous solids are more resistant to the presence of water vapor but tend to absorb less CO₂ [4,6]. Furthermore, the high temperatures required for the regeneration of these materials after being used for CO₂ adsorption is also a major obstacle [4,6]. To address the limitations presented by inorganic absorbents, during the last decade, the use of bio-based absorbents for CO₂ capture has been extensively investigated, emphasizing cellulose nanofibril (CNF) [7,8].

The chemical modification of CNF for CO₂ capture using silane coupling agents is considerably investigated [7,8,9,10,11,12,13]. Gebald et al. (2014) was the first to suggest the use of nanocelluloses as a solid support for CO₂ adsorption, achieving an adsorption of 1.39 mmol CO₂ g⁻¹ [14].

Similarly, nanocellulose adsorbents were modified using aminosilanes, namely N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, phthalimide (1,3-dihydro-1,3-dioxoisoindole), N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, (3-trimethoxysilylpropyl) diethylenediamine, and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS), achieving a CO₂ adsorption of between 0.5 and 5 mmol CO₂ g⁻¹ [7,8,9,10,11,12,13]. Valdebenito et al. (2018) reported the synthesis of nanocellulose thin films for CO₂ adsorption from corn husks, oat hulls, and kraft pulp, and modified them using 3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, [3-(trimethoxysilyl) propyl] ethylenediamine, and (3-trimethoxysilylpropyl) diethylenetriamine. The nanocellulose film derived from kraft pulp, modified using [3-(trimethoxysilyl) propyl] ethylenediamine, had the highest CO₂ adsorption capacity of 2.11 mmol g⁻¹ at room temperature and atmospheric pressure. It is worth noting that this modified nanocellulose thin film has the highest amine content, but the lowest surface area, demonstrating that chemisorption was the dominant adsorption type [8].

Due to the significant effect of amino loading on CO₂ adsorption capacity, it would be valuable to further explore how this variable affects the maximum CO₂ reversible adsorption capacity of the aerogel (CO₂ adsorption and desorption stages) and its lifespan.

The main objective of this study was to evaluate the maximum reversible CO₂ adsorption capacity, stability, and lifespan of amino-functionalized nanocellulose aerogels as CO₂ adsorbent materials. By using the decay pressure technique, which allows measurements at pressures higher than atmospheric, to assess the effect of pressure on CO₂ adsorption, this approach provides valuable insights into the potential of these materials for long-term use in CCS and CCU technologies.

2. Materials and Methods

The nanocellulose hydrogels were obtained from commercial cellulose (kraft pulp), N-[3-(Trimethoxysilyl) propyl] ethylenediamine (DAMO), (2,2,6,6,6-tetramethylpiperidinyl-1-oxyl) TEMPO, NaClO, NaClO₂, NaOH, glacial acetic acid, and ethanol, which were purchased from Merck/Sigma Aldrich, Santiago de, Chile.

2.1. Synthesis of Nanocellulose Aerogels

The TEMPO-mediated oxidation of cellulose suspension was carried out according to Saito et al. (2005) with some modifications [15]. The oxidized fibers were dispersed in deionized water with a pulp concentration of 1 wt.% and then homogenized with a high-pressure homogenizer (NS1001L PANDA 2K-GEA) 8 times at a pressure drop of 800 bar to produce nanocellulose hydrogels. To produce nanocellulose aerogels from hydrogels, the freeze-dried technique of an aqueous suspension of nanocellulose was used. This suspension was poured into a mold, then the samples were frozen at –21 °C for at least 24 h to be lyophilized (FreeZone 6, Labconco) at –41 °C and 0.01 mbar for 12–30 h depending on the amount of water to remove. At the end of this process, nanocellulose aerogels were obtained.

2.2. Nanocellulose Aerogel Silylation with N-[3-(Trimethoxysilyl) Propyl] Ethylenediamine (DAMO)

The obtained aerogels were functionalized with 3 different amino group loads, i.e., 4.62, 9.24, and 13.87 mmol portions of DAMO (see Figure 1), which were hydrolyzed in a 50 mL mixture of ethanol/water (95/5, wt.%). The pH was adjusted to 3.5 by adding acetic acid, and the solution was stirred at room temperature for 2 h. Then, nanocellulose aerogel was immersed into this solution. The solution was stirred for 2 h at room temperature. This protocol was repeated for the 3 amino group loadings. The modified nanocellulose aerogels were thoroughly washed with ethanol (soxhlet extraction with ethanol) before being dried at room temperature in a closed system and stored in a desiccator to avoid humidity [8].

2.3. Characterization

The analysis of functional groups in the nanocellulose aerogel was carried out through infrared spectroscopy. The IR measurements were performed with an Agilent Tensor 27 instrument (Malaysia) in Fourier transform mode (FTIR). The Agilent MicroLab PC software (version IQ/OQ, 21 CFR Part 11 compliant) was used for data acquisition. A total of 40 scans were collected across a spectral range of 400 to 4000 cm⁻¹ [7,8].

The carbon, hydrogen, and nitrogen contents in the nanocellulose aerogels were quantified using an EA 3000 Elemental Analyzer, from Perkin Elmer, Chile. The analysis was performed in triplicate for each sample. The samples were weighed on a microbalance (Sartorius CP2-P) between 1.5 and 2.5 mg for each sample. Each sample was subsequently placed in a previously tared tin capsule. The elemental analyzer was calibrated using 5-level L-cysteine (0.1, 0.25, 0.5, 1.0, and 2.0 mg), and the encapsulated sample was loaded into the equipment. The 21 CFR software (version Part 11) was used for data acquisition.

Thermo-gravimetric analysis was performed using a STA 6000 apparatus from Perkin Elmer Chile, at a heating rate of 10 °C min⁻¹ up to 800 °C under the nitrogen atmosphere (flow rate = 90 mL min⁻¹). About 30 mg of each sample were analyzed. The Pyris software (version 13.4.0.0036) was used for data acquisition [7,8,13].

Scanning electron microscopy (VP-SEM) analysis was performed. The samples were adhered to the sample holder with double-sided carbon tape. The sample was visualized using a backscatter detector (BSE) in variable pressure without any further sputtering under the following parameters: 10 KV energy, 20 Pa pressure, and WD 10 mm in a scanning electron microscope (Hitachi SU3500, Tokyo, Japan). The images were acquired and analyzed with Hitachi software controller and Image J 1.53k Java 1.8.0_172 Software (Wayne Rasband and contributors, National Institutes of Health, Betheda, MA, USA) [8,9,13].

For the X-ray photoelectron spectroscopy (XPS) analysis, an Axis Ultra DLD electron spectrometer was used. Survey scans were recorded using monochromated Al Kα irradiation with a 50 W, 0.1 eV step, and 160 eV analyzer pass energy. Analysis = 700 um × 300 um; charge neutralizer, ON; narrows scans (elemental quantification and peak fitting), pass energy = 20 eV; energy calibration, C 1s C-(C, H) component @285.0 eV [7,8].

2.4. CO₂ Adsorption Measurements

The CO₂ adsorption was measured through CO₂ adsorption isotherms at 273 K (relative pressure: 0.00002–0.02) using Micromeritis Tristar II 3020 equipment, with samples degassed at room temperature for 48 h. The adsorption isotherms were adjusted using the BET, Langmuir, Freundlich, and Temkin models (see

Table 1. Isotherm models.

Isotherm	Equation
BET	${\displaystyle \frac{X}{\left(1-x\right)V}}={\displaystyle \frac{1}{cV}}+{\displaystyle \frac{c-1}{cV}}x$
Langmuir	${\displaystyle \frac{1}{V}}={\displaystyle \frac{1}{KVP}}+{\displaystyle \frac{1}{V}}$
Freundlich	θ = KP1/n
Temkin	θ = A ln (B.P)

) [7,8,13].

The pressure decay technique was used to determine CO₂ adsorption capacity. The dual-chamber gas sorption cell followed the method of Koros et al. (1976). Before measurements, 0.7 g⁻¹ g of the sample were weighed and dried for 1 h at 70 °C (343 K). CO₂ sorption experiments were carried out at 25 °C (298 K) and at 4 bar. CO₂ sorption capacity was calculated using Equations (1) and (2) [16].

$$n_{C{O}_2}={\displaystyle \frac{P_i{V}_{gc}}{Z_{\left(P_i,T_i\right)}R{T}_i}}-{\displaystyle \frac{P_{eq}\left(V_t-V_p\right)}{Z_{\left(P_{eq},T_{eq}\right)}R{T}_{eq}}}$$

(1)

$$w_{C{O}_2/g}={\displaystyle \frac{n_{C{O}_2}M}{W_s}}$$

(2)

Here, wCO₂/g is the weight of gas adsorbed by the sample, P_i and T_i give the pressure and the temperature in the gas chamber, respectively, and those parameters at equilibrium are represented as P_eq and T_eq; V_gc is the gas chamber’s volume, V_p is the volume of the sample, and V_t is the total volume of the sorption cell. The coefficient of compressibility “Z” for CO₂ was obtained via the Span–Wagner equations of state [16,17].

2.5. Nanocellulose Aerogel Adsorption/Desorption Cycle Study (TGA)

Adsorption/desorption cycle analysis was performed on a STA 6000 apparatus from Perkin Elmer; it used a flow of 50 mL min⁻¹ of N₂ (99.999% purity) for 24 h for sample degassing. Then, a 20 mL min⁻¹ of CO₂ flow (99.999% purity) was added and was maintained for 30 min. Subsequently, N₂ conditions were resumed for 30 min. The analysis was carried out at room temperature 298 K 20 times.

3. Results

3.1. Infrared Spectroscopy Analysis (FTIR)

The FTIR spectra of both nanocellulose aerogel unmodified and modified with di-aminosilane are shown in Figure 2. The spectrum of unmodified aerogel (A0) exhibited typical bands for cellulose, such as O-H stretching at 3200 cm⁻¹, C-H stretching at 2900 cm⁻¹, CH₂ symmetric bending at 1400 cm⁻¹, and O-H and C-H bending as well as C-C and C-O stretching at 1380, 1310, and 1250 cm⁻¹ respectively.

The spectrum of aerogel modified (A1, A2, and A3) showed the successful grafting of di-amino silane on nanocellulose. The band at 2900 cm⁻¹ was assigned to C-H stretching, the O-H stretching at 3200 cm⁻¹ was replaced for the band at 3300 cm⁻¹ assigned to N-H stretching. Also, the band at 1700 cm⁻¹ assigned to N-H flexion and the appearance of signals associated with vibrations from silicon-based linkages were observed at 1240 cm⁻¹, and between 1180 and 700 cm⁻¹ (Si-OH, Si-O-C) [8,9,13,14].

3.2. Elemental Analysis (C, H, and N)

The elemental distribution over the surface was confirmed by elemental analysis (C, H, and N) and are shown in

Table 2. Nanocellulose aerogel elemental analysis.

Sample	C wt.%	H wt.%	N wt.%	O wt.%
A0	39.25	5.68	0.07	55.0
A1	39.80	6.13	0.35	53.7
A2	39.77	6.40	1.55	52.8
A3	39.97	6.75	2.74	50.5
STD *	71.64	6.64	10.42	11.3

* Calibration standard, L-cysteine.

. The constituents C, O, and N show that the grafting of the amino group took place in the polymer. From Table 2, it can be observed that increasing the amino group content in the aerogel led to an increase in the nitrogen percentage. Interestingly, a linear relationship was observed between the amino group content and nitrogen content due to the grafting of amino groups onto the nanocellulose matrix.

3.3. Nanocellulose Aerogel Thermo-Gravimetric Analysis (TGA)

The functionalized aerogels (A1, A2, and A3) were compared to unmodified A0. The weight loss and derivative of weight loss versus temperature plots (Figure 3) for A0 show multiple degradation events. The onset of thermal degradation is practically the same for A0 and A1 aerogels.

All sample thermograms demonstrate two weight loss events preceding the main decomposition step, as can be seen from Figure 3. The first degradation (at between 90 and 100 °C) may be attributed to loss of some bound volatile material, probably residual water. Similarly, the peak around 240 °C in all aerogel’s samples, correspond to hemicelluloses that were not removed during the pulping process. Finally, the peak around 320° in all samples corresponds to the degradation temperatures of the cellulose [7,8,9,14]. The developed nanomaterials (biodegradable materials) exhibited adequate thermal stability up to 240 °C, which is one of the pre-requisites of the CO₂ capture materials at the industrial scale.

3.4. Scanning Electron Microscopy (VP-SEM)

SEM images of nanocellulose aerogels modified with DAMO A0 and A1 (A1; like A2 and A3 images) are shown in Figure 4. The unmodified aerogel exhibited a random pore structure and disorderly cross-linked CNFs were observed. After amino group grafting, many planar structures with individual CNFs irregularly attached to the cellulose sheet were observed (Figure 4b). These results are in accordance with previously reported studies in the literature [9,13].

3.5. X-Ray Photoelectron Spectroscopy (XPS)

XPS measurements were next performed to further analyze the chemical composition and surface electronic state of the CNF aerogel samples (Figure 5). The binding energy (BE) peaks from the N1s at 398–402 eV appeared in the A1, A2, and A3 samples. The N1s spectrum of modified CNF aerogels (Figure 5A–C) can be fitted to three peaks with the BE at 400 (R₂NH and RNH₂) and 402.0 (R₃N) eV, demonstrating that the DAMO was anchored to the surface of CNF aerogels [7,8,13].

The N1s deconvolution spectrum reveals that as the amino group loading in the aerogels increases (Figure 5A–C), the signal intensity for the R₃N and R₂NH groups decreases, while the intensity of the RNH₂ group increases. Consequently, aerogel A3 exhibits the highest percentage of NH₂ groups grafted onto the nanocellulosic matrix, making them available for CO₂ capture.

Another finding observed in the N1s deconvolution spectra is that as the amino group content increases (Figure 5A–C), the peak corresponding to the RNH₂ group shifts toward the higher binding energy region. This shift can be attributed to changes in local electronegativity caused by the formation of the carbamate ion. The formation of this ion alters the electronic density, resulting in the RNH₂ peak moving to higher binding energies.

Additionally, this phenomenon may be influenced by polarization effects, which enhance the reactivity of the RNH₂ group toward CO₂. This increased reactivity contributes to a higher CO₂ adsorption capacity in aerogels modified with a greater amino group content. Such behavior could be explained by the overlap of C-N atomic orbitals. In other words, changes in the amino group charge induce alterations in the electronic density of the covalent bond, facilitating CO₂ chemisorption and improving the aerogel’s performance.

3.6. CO₂ Absorption Isotherms at 273 K

CO₂ adsorption isotherms were carried out at 273 K for all samples studied and were classified as type 1 (see Figure 6) in the Brunauer–Deming–Deming–Teller (BDDT) isotherm classification, corresponding to microporous solids. This classification indicates monolayer adsorption, typical of microporous materials with strong adsorbate–adsorbent interactions. To calculate the CO₂ adsorption capacity, the experimental data were fitted to Langmuir, BET, Freundlich, and Temkin models (see Supporting Information). The deviation from the typical type 1 isotherm behavior observed in Figure 6 at higher relative pressures, resulting from volume swelling, has been corrected in the linearized adsorption isotherms of CO₂ (Langmuir and BET), as presented in the Supplementary Information (Supporting Information).

As the relative pressure increased, the CO₂ adsorption capacity reached a maximum at approximately 0.4 bar (micropore filling) and subsequently decreased gradually. This phenomenon has also been reported in other studies and can be attributed to the nonlinearity of the apparent CO₂ density with temperature and pressure [18,19].

Table 3. Isotherms parameters.

Isotherms	Parameters	A1	A2	A3
BET	S (g/cm3 STP)	0.2216	0.1843	0.2037
	Y (g/cm3 STP)	–0.0044	–0.0034	–0.0036
	R	0.9980	0.9982	0.9981
	Qm (cm3/g STP)	4.6032	5.5292	4.997
Langmuir	S (g/cm3 STP)	0.1695	0.1341	0.1576
	Y (g/cm3 STP)	–1.2550	–0.3500	–1.0360
	R	0.9988	0.9997	0.9987
	Qm (cm3/g STP)	5.8963	7.4543	6.3445
Freundlich	S	6.8946	6.5043	7.4617
	R	0.7788	0.8586	0.7864
	Qm (cm3/g STP)	2.7227	3.2204	3.1092
Temkin	S (mmHg−1)	268.6050	94.1404	441.948
	R	0.7887	0.8860	0.8080
	Qm kJ mol−1 (cm3 g−1 STP)	1.2021	0.8736	1.1657

shows some parameters derived from the adsorption isotherms for nanocellulose aerogels at 273 K. Qm corresponds to the amount adsorbed in cm³ g⁻¹, calculated for each linearized isotherm model under standard temperature and pressure conditions. Based on this parameter, the adsorbed amount Xo (mmol g⁻¹) from

Table 4. Nanocellulose aerogels: BET Area, Qm ^a and X0 ^b.

Sample	BET Area (m2 g−1)	Qm (cm3 g−1)	X0 (mmol g−1)
A0	94.00	–	–
A1	154.00	4.60	0.21
A2	154.80	5.52	0.25
A3	154.30	4.99	0.22

^a CO₂ amount absorbed under standard temperature and pressure conditions (cm³ g⁻¹); ^b CO₂ amount adsorbed under standard temperature and pressure conditions (mmol g⁻¹).

is determined using the ideal gas law equation. R is the correlation coefficient, indicating how well our experimental data fit each model. Based on these criteria, the isotherm classification (BDDT), and the reaction mechanism, the most suitable model is selected. The adsorption isotherms at 273 K are kept at low temperatures to measure the amount of CO₂ that can be adsorbed by a material at different relative pressures. At these temperatures, the adsorption is physical, allowing for the evaluation of the material’s maximum adsorption capacity without the effects of chemical reactions at higher temperatures. This technique is very useful for characterizing porous materials and comparing their adsorption capacity under controlled conditions. However, the results obtained at these temperatures may not represent the material behaviors at temperatures more relevant to practical applications, such as CO₂ capture in environmental or industrial conditions.

For physical adsorption in multilayers, the BET model was used to calculate the amount of CO₂ adsorbed. Nanocellulose aerogels modified with different amino-silane loads, i.e., 4.62, 9.24, and 13.87 mmol, showed CO₂ adsorptions of 0.21, 0.25, and 0.22 mmol CO₂ g⁻¹. At 273 K (see Table 4), this adsorption was due to physisorption. No significant differences were observed in the adsorption of the three aerogels studied. Aerogel A2 showed slightly higher CO₂ adsorption, due to having a slightly larger surface area than the others, as can be seen in Table 4.

Baraka et al. (2024), indicate that most cellulose nanofiber aerogels reported in the literature have a BET surface area ranging from 7.1 to 335 m² g⁻¹. Therefore, our results are in close agreement (see Table 4, 154 m² g⁻¹) when obtained through the same drying method (freeze-drying) and functionalization method (liquid phase) [20].

3.7. CO₂ Absorption Capacity Through Pressure Decay Technique at 298 K and 4 Bar

Figure 7 shows CO₂ adsorption capacities of each aerogel (A1, A2, and A3). The values obtained for the DAMO-modified nanocellulose aerogels ranged from 3.17 to 7.86 mmol CO₂ g⁻¹ at 25 °C, proving a significant increase in CO₂ adsorption with an increase in amino group content in the nanocellulose aerogel. These results are promising. The literature reports CO₂ adsorption capacities of amine-modified cellulose nanofiber (CNF) aerogels ranging from 1.39 to 2.4 mmol g⁻¹ (

Table 5. CO₂ adsorption capacity of various cellulosic materials.

Cellulosic Materials	Functionalization	CO2 Adsorption Capacity (mmol g−1)	Reference
CNC and CNFs	AEAPDMS	2.36	[7]
CNFs	DAMO	2.16	[8]
CNFs	APS	1.91	[9]
CNFs	APMDS	1.02	[10]
CNC	AEAPMDS	2.63	[11]
CNC	AEAPMDS	1.59	[12]
CNFs	Phthalimide	5.20	[13]
CNFs	DAMO	7.86	This work

). These values are lower than those achieved in the present work at the same adsorption temperature (see Table 5). Sepahvand et al. (2020) obtained 5.2 mmol g⁻¹ of CO₂ adsorption from CNF aerogels modified with phthalimide (1.5% phthalimide content) under the same freeze-drying and liquid phase functionalization methods, using a nonlinear amine (phthalimide). However, this value is lower than the adsorption capacities achieved by the samples A2 and A3, which attained CO₂ capture values of 5.98 and 7.86 mmol g⁻¹, respectively.

Moreover, it can be observed that the obtained results are higher than the adsorption capacities of other typical adsorbents used in CO₂ capture. For example, the APTS-modified MCM-41 reached a value of 1.33 mmol CO₂ g⁻¹ [21]. Furthermore, MCM-41 exhibited a higher BET surface area (1602 m² g⁻¹) than the nanocellulose aerogels studied in the present work.

The high adsorption values obtained are mainly explained by the chemisorption mechanisms between the amino groups and CO₂ molecules. The reaction of CO₂ with DAMO involves a direct reaction between the carbon atom of CO₂ and the nitrogen atom of the amine, forming a covalent bond [22,23]. Therefore, the chemical modification of nanocellulose with amine moieties enhances the adsorption capacity by providing many active binding sites that promote interactions between the aerogel and CO₂ molecules [22,23]. The molar ratio of amine introduced could also significantly influence the CO₂ adsorption efficiency of modified nanocellulose aerogels. A high concentration of amine promotes a high adsorption capacity [24].

This trend is consistent with the results obtained from the XPS analysis proving the higher % NH₂ where a linear tendency was observed. With a higher amino group loading, there was an increased nitrogen content (amino group according to the supported amine reaction mechanism via carbamate ions) that was grafted onto the nanocellulosic matrix. Additionally, there was also greater CO₂ adsorption capacity measured at room temperature. Regarding our results, a significant increase in CO₂ adsorption is observed with a slight increase in DAMO loading (4.62, 9.24, and 13.87 mmol of DAMO, respectively). The pressure decay technique can be used at different temperatures and pressures, allowing for a simulation closer to real operating conditions. It is useful for studying the kinetics of adsorption and desorption and the adsorption capacity [20].

Unlike most studies reported in the literature, this research uses the pressure decay technique to determine the maximum reversible adsorption CO₂ capacity of amino-functionalized CNF aerogel. This method allows for pressures above atmospheric levels, making it possible to evaluate how increased pressure affects the adsorbent capacity at room temperature. This approach more accurately simulates the higher-pressure conditions typical of industrial carbon capture and utilization processes. The high CO₂ adsorption capacity of sample A3 (with the biggest amino group load), compared to literature review studies (see Table 5), is largely attributed to raising the pressure from around 1 bar to 4 bar [25]. The higher CO₂ pressure enhanced CO₂ density and its interaction with amino groups on the aerogel surface, promoting efficient carbamate ion formation and enabling greater CO₂ capture.

3.8. Nanocellulose Aerogel Lifespan

The study of CO₂ adsorption/desorption cycles was conducted using the aerogel that exhibited the highest CO₂ adsorption, specifically sample A3. Figure 8 shows that the CO₂ desorption was fast and was complete after 30 min. A total of 95 wt.%. of the absorbed CO₂ was desorbed during 20 adsorption/desorption cycles (reversible adsorption) measured at room temperature (298 K) in the thermobalance (TGA). It can be concluded that this number of cycles is likely higher since after 20 cycles, the aerogel continued to exhibit the same behavior as in the first cycle. Gebald et al. (2011) and Zhu et al. (2011) reported that CO₂ desorption was fast and was complete after 30 min for a similar substrate. More than 85% of the CO₂ was desorbed within 19 min at a packed bed temperature of below 80 °C [7,13,14].

Figure 9 shows the sorption/desorption test (pressure decay technique) using the same sample (A3) across seven cycles. The sample was regenerated by heating to 70 °C for 20 min at the end of each sorption test. The results prove that the sample retains the CO₂ sorption capacity for up to seven cycles; this could likely be higher as the retained sample exhibited the same sorption capacity as in the first cycle, demonstrating the recyclability of the synthesized material. These results agree with results obtained from the adsorption/desorption cycles at 298 K determined by thermogravimetric analysis. Both techniques demonstrated the stability and reusability of the synthesized materials, which is one of the pre-requisites of the CO₂-capture materials at the industrial scale.

4. Conclusions

This study highlights the potential of DAMO-modified nanocellulose aerogels for scalable CO₂ capture and storage applications. The predominant chemisorption mechanism, characterized by the interaction between CO₂ molecules and amino groups to form covalent bonds, was found to be highly effective for CO₂ adsorption. A significant increase in CO₂ adsorption was observed with a slight increase in DAMO loading (4.62, 9.24, and 13.87 mmol of DAMO, respectively), ranging from 3.17 to 7.86 mmol CO₂ g⁻¹ at 298 K (25 °C) and 4 bar. The use of the decay pressure technique allowed the study to examine the effect of pressure on CO₂ adsorption at pressures higher than atmospheric conditions, thus simulating real industrial conditions more closely.

The nanocellulose aerogels demonstrated excellent thermal stability at up to 240 °C and consistent performance across adsorption/desorption cycles, desorbing 95% of the adsorbed CO₂ over 20 cycles at room temperature. This stability and reusability underscore their practicality for repeated use in industrial CO₂ capture processes. These results support the feasibility of utilizing these biodegradable materials in scalable CO₂ capture technologies, making them promising candidates for sustainable and efficient carbon capture alternatives.