A Simple Regeneration Process Using a CO₂‐Switchable‐Polarity Solvent for Cellulose Hydrogels

Abstract

Cellulose is one of the main components of plant cell walls, abundant on earth, and can be acquired at a low cost. Furthermore, there has been increasing interest in its use in environmentally friendly, carbon‐neutral, sustainable materials. It is expected that the applications of cellulose will expand with the development of a simple processing method. In this study, we dissolved cellulose in aqueous N‐butyl‐N‐methylpyrrolidinium hydroxide solution ([C₄mpyr][OH]/H₂O) and investigated the cellulose regeneration process based on changes in solubility upon application of CO₂ gas. We investigated the effect of transformation of the anion chemical structure on cellulose solubility by flowing CO₂ gas into [C₄mpyr][OH]/H₂O and conducted pH, FT‐IR, and ¹³C NMR measurements. We observed that the changes in anion structure allowed for the modulation of cellulose solubility in [C₄mpyr][OH]/H₂O, thus establishing a simple and safe cellulose regeneration process. This regeneration process was also applied to enable the production of cellulose hydrogels. The hydrogel formed using this method was revealed to have higher mechanical strength than an analogous hydrogel produced using the same dissolution solvent with the addition of a cross‐linker. The ability to produce cellulose‐based hydrogels of different mechanical properties is expected to expand the possible applications.

The effect of transformation of the anion chemical structure on cellulose solubility by flowing CO₂ gas into aqueous pyrrolidinium hydroxide solutions was investigated. The changes in anion structure allowed for the modulation of cellulose solubility in aqueous pyrrolidinium hydroxide solutions, thus establishing a simple and safe cellulose regeneration process. This regeneration process was also applied to enable the production of cellulose hydrogels.

Introduction

Environmental pollution and climate change pose threats to both the quality of life on earth and human society. To minimize these threats, there is a demand for new environmentally friendly materials and processes. The use of cellulose, the primary component of plant cell walls, as a material is attracting great interest. Cellulose is produced by plants on a massive scale, approximately 1.5×10¹² tons annually, ^[1] and cellulose‐derived materials might be conveniently recycled as cellulose is biodegradable. ^[2] Moreover, it can be sourced from non‐edible materials, resulting in low environmental impact, making it a promising carbon‐neutral sustainable material.[ ³ , ⁴ ] One of the ways to effectively utilize cellulose is through the creation of cellulose hydrogels. Cellulose hydrogels offer adjustable physicochemical properties through cross‐linking, making them suitable for various applications in fields such as medicine and sensing.[ ⁵ , ⁶ ] Additionally, there is a need for efficient absorption and conversion of CO₂, which is a major contributor to climate change. ^[7] However, current CO₂ absorption materials face challenges such as high costs and synthetic difficulties, and approaches for CO₂ utilization exhibit low conversion efficiencies. ^[8]

To produce hydrogels from cellulose, it′s essential to control the dissolution and regeneration processes of cellulose. However, cellulose is insoluble in water and most common organic solvents due to numerous intramolecular and intermolecular hydrogen bonds. ^[9] Conventional cellulose dissolution processes, such as the viscose process and cuprammonium process, involve the use and release of toxic substances like carbon disulfide and hydrogen sulfide.[ ¹⁰ , ¹¹ ] Therefore, ionic liquids (ILs) and deep eutectic solvents (DES) have garnered attention as cellulose solvents.[ ¹² , ¹³ , ¹⁴ ] ILs offer high thermal stability and can dissolve cellulose, and they can be used as environmentally friendly solvents by extracting and regenerating cellulose.[ ¹⁵ , ¹⁶ , ¹⁷ ] Additionally, while DES have low solubility for cellulose, they dissolve lignin from lignocellulose with very high efficiency, making them useful for the separation of biomass components. However, the dissolution of cellulose in ILs has often required harsh conditions and yielded undesirable by‐products. For example, 1‐butyl‐3‐methylimidazolium chloride ([C₄mim]Cl) is reported to dissolve cellulose at temperatures above 80 °C but exhibits significantly reduced solubilization of cellulose in the presence of small amounts of water, necessitating an energy‐intensive water separation step for wet cellulose feedstocks prior to dissolution.[ ¹⁸ , ¹⁹ ] Alternatively, sodium hydroxide (NaOH) aqueous solution can dissolve cellulose in the presence of water but requires a low temperature of under 0 °C. ^[20] N‐Methylmorpholine‐N‐oxide (NMMO) also dissolves cellulose in the presence of water but comes with various drawbacks such as cellulose chain degradation and reduced functionality of target products.[ ²¹ , ²² ] To address these issues, recent efforts have focused on developing cellulose solvents that can dissolve cellulose under milder conditions, such as aqueous tetrabutylphosphonium hydroxide (TBPH) solutions, which can dissolve cellulose in the presence of a significant amount of water. ^[23]

Additionally, CO₂‐switchable solvents are gaining attention as solvents that minimize environmental impact as much as possible. CO₂‐switchable solvents are substances whose properties change with the absorption or desorption of CO₂ gas. Waste CO₂ gas is particularly low‐cost and environmentally friendly as a trigger because it can be used at 1 atmosphere, is non‐toxic and non‐flammable, and can be easily removed. Reported CO₂‐switchable solvents can be classified into three types: SPSs (Switchable Polarity Solvents), which change solvent polarity; SHSs (Switchable Hydrophilicity Solvents), which change hydrophilicity; and SW (Switchable Water), which changes ionic strength. ^[24] The dissolution and regeneration process of cellulose using DBU (1,8‐diazabicyclo [5.4.0] under‐7‐ene) and DMSO, one type of CO₂‐switchable solvent, has also been reported. ^[25] This process has a lower environmental impact compared to traditional methods, as it does not involve side reactions or by‐products. The cellulose carbonate anion obtained through this dissolution process is nucleophilic, enabling functional group modification through cyanoethylation without using catalysts and avoiding side reactions. ^[26] Additionally, further development has been conducted to demonstrate a possible application for freshness monitoring using fluorescence intensity differences by modifying the cellulose, in this type of switchable solvent system, with functional groups that respond to amines. ^[27]

Upon dissolution of cellulose, further processing is required to generate a hydrogel. Methods for creating hydrogels from cellulose solutions can be broadly categorized into two types: chemical cross‐linking and physical cross‐linking. Chemical cross‐linking involves the formation of covalent or ionic bonds between functional groups through the addition of cross‐linking agents. Physical cross‐linking, on the other hand, is driven by interactions like hydrogen bonding, hydrophobic interactions, and electrostatic interactions, allowing for the suppression of toxic chemical use and release. ^[28] Physical cross‐linking offers an environmentally friendly gelation process since it avoids the need for additional cross‐linking agents. ^[29]

In this research study, the aim is to establish a cellulose regeneration process that is convenient and environmentally benign, and to further apply this for the formation of hydrogels. We used aqueous N‐butyl‐N‐methylpyrrolidinium hydroxide solution (here‐after [C₄mpyr][OH]/H₂O) as the cellulose solvent. We have previously reported that [C₄mpyr][OH]/H₂O exhibits a cellulose solubility of 20 wt % without the need for heating, and that the dissolution occurs relatively rapidly. ^[30] This is due to the ability of hydroxide ions to form of hydrogen bonds between them and the hydroxyl groups of cellulose. Subsequently, in order to gel the solution, it is necessary to modify the cellulose solubility of [C₄mpyr][OH]/H₂O. ^[31] It has been reported that cellulose dissolved in an aqueous TBPH solution can be gelled by the addition of propylene carbonate. ^[32] This is thought to occur because propylene carbonate reacts with OH⁻ resulting in the formation of 1,2‐propandiol and CO₂. This transformation of the hydroxide ions, which play a critical role in cellulose dissolution, into bicarbonate ions is believed to result in a consequential loss of cellulose solubility, and thus gelation occurs. ^[32] In this study, rather than employing propylene carbonate as a stoichiometric reagent consumed in the gelation process (with formation of a diol by‐product), here we investigated the reactivity of CO₂ gas directly with [C₄mpyr][OH]/H₂O through pH, FT‐IR, and ¹³C NMR measurements. Subsequently, we used the cellulose regeneration process established in this work to prepare cellulose hydrogels and investigated changes in the crystallinity of cellulose using XRD measurements. Finally, we assessed the mechanical strength of the cellulose hydrogels through compression tests and compared them to those produced using different processes. We note here that the reported result for cellulose solubility of 20 wt % was achieved with a starting [C₄mpyr][OH]/H₂O solution of 50 wt % (i. e. 1 : 1 mass ratio of [C₄mpyr][OH] and H₂O), ^[30] and [C₄mpyr][OH]/H₂O with the same 50 wt % concentration was used throughout the current study.

Results and Discussion

Chemical Characterization

To investigate the structural changes in hydroxide ions, the change in pH value of [C₄mpyr][OH]/H₂O with respect to the exposure to CO₂ was examined. Figure 1 illustrates the relationship between the duration of CO₂ exposure and pH values. The initial pH value before CO₂ flow was measured. It was found that the pH value was above 14, revealing that a highly alkaline aqueous solution promotes the cleavage of hydrogen bonds in cellulose. As the duration of CO₂ flow progressed, the pH of [C₄mpyr][OH]/H₂O gradually decreased from 14. After 45 min, the pH value dropped significantly to 9. The introduction of CO₂ resulted in a decrease in the pH value of [C₄mpyr][OH]/H₂O, indicating a reduction in the concentration of hydroxide ions.

Figure 1.

Relationship between the duration of CO₂ flow and pH values for [C₄mpyr][OH]/H₂O. The pH measurements were performed in triplicate at each CO₂ exposure time. The error bars represent maximum/minimum values.

The FT‐IR spectra are complicated by the overlap of various bands from possible species in the solution, notably (i) H₂O (~1640 cm⁻¹, assigned to the possible combination of the OH stretch fundamental and bend overtone), ^[33] (ii) HCO₃ ⁻ (~1634 cm⁻¹, assigned to the asymmetric stretch of the CO₂ moiety; 1364 cm⁻¹, assigned to the symmetric stretch of the CO₂ moiety; and 1320 cm⁻¹, assigned to the bend of the COH moiety), ^[34] and (iii) CO₃ ²⁻ (1428 and 1378 cm⁻¹, assigned to the asymmetric stretch of the CO moiety). ^[34]

Figure 2 presents the FT‐IR spectra of [C₄mpyr][OH]/H₂O at varying durations of CO₂ flow. In the spectra shown in Figure 2, the increase in band intensity and then splitting into two bands of the signal in the range of 1400–1300 cm⁻¹, along with the apparent increase in intensity of the band centered at approximately 1650 cm⁻¹ is suggestive of the gradual production of HCO₃ ⁻ (bicarbonate). However, the presence of CO₃ ²⁻ cannot be excluded. Further clarification of the speciation is achieved with ¹³C NMR spectroscopy (vide infra).

Figure 2.

Changes in FT‐IR spectra of [C₄mpyr][OH]/H₂O with CO₂ exposure time.

The ¹³C NMR spectra of [C₄mpyr][OH]/H₂O at each duration of CO₂ flow are shown in Figure 3. After 5 min, a peak tentatively attributed to a C=O functionality, which was not present before reacting with CO₂, was observed at 166 ppm. As the duration of CO₂ flow progressed, this peak based on a carbonyl group shifted to a higher magnetic field, and after 55 min, it was observed at 159 ppm. ^[35] Previous research has reported ¹³C NMR measurement results for the exposure of an IL/DMSO/water mixture to CO₂. In these previous results, the peaks corresponding to CO₃ ²⁻ and HCO₃ ⁻ were observed at 167 ppm and 159 ppm, respectively. ^[35] In this study, it is believed that the peaks can be assigned to firstly CO₃ ²⁻ and then HCO₃ ⁻ as the duration of CO₂ flow progressed. Gradual upfield shift for both the CO₃ ²⁻ and HCO₃ ⁻ peaks are attributed to pH changes, as pH changes are known to influence chemical shifts. Generally, when OH⁻ is consumed, resulting in a lowering in pH, lower chemical shifts are observed. In addition, peaks originating from pyrrolidinium cations were observed at chemical shifts lower than 155 ppm, but no significant changes were observed with time (Figure S2).

Figure 3.

Changes in ¹³C NMR spectra of [C₄mpyr][OH]/H₂O with CO₂ gas exposure time.

Taken together, along with knowledge of the typical speciation of CO₂ derived species in aqueous solution as a function of pH in a typical Bjerrum plot (see Figure S2), ^[36] the FT‐IR data and NMR data indicate a stepwise change in composition of the solution with CO₂ exposure duration. The counter‐anions of the pyrrolidinium cations undergo reaction with CO₂.

In the initial phase, some proportion of the OH⁻ reacts with CO₂ to form CO₃ ²⁻ (favored at a pH higher than the pK _a of HCO₃ ⁻, i. e. 10.3). Then, as the reaction continues, the pH decreases as more OH⁻ is consumed, and HCO₃ ⁻ becomes the dominant CO₂‐derived species. Thus we propose that the anionic species of the salt [C₄mpyr][OH] underwent a transformation from hydroxide ions to bicarbonate ions through the absorption of CO₂. It is also suggested that there were no structural changes in the pyrrolidinium cations during this process (see Figure S3). ^[37]

Although the quantitative conversion of OH⁻ to HCO₃ ⁻ is no unequivocally demonstrated here, we note that observation of the water content change during the exposure to CO₂ (see Figure S4) is suggestive of such a change (50.0 wt. % for [C₄mpyr][OH]/H₂O, expected to reach 43.9 wt. % upon quantitative conversion to [C₄mpyr][HCO₃]/H₂O). Also, estimating the pH for a solution containing only [C₄mpyr][HCO₃], the amphiprotic HCO₃ ⁻ is expected to result in a pH of ~8.3 (pH=1/2(pK _a(H₂CO₃)+pK _a(HCO₃ ⁻))=1/2(6.4+10.3)). This is in broad agreement with the pH reached after prolonged exposure to CO₂, see Figure 1. Thus, we propose that after CO₂ exposure, the aqueous solution can be considered to contain [C₄mpyr][HCO₃], with some minor proportion of [C₄mpyr][OH] remaining.

Having established the conversion of the anion, we address whether the solubility of cellulose in [C₄mpyr][OH]/H₂O can be altered by the influx of CO₂ gas. As a result of the change in the anion in the solution from OH⁻ to HCO₃ ⁻, it is expected that the solubility of cellulose will decrease, thus providing a facile method for cellulose regeneration from the [C₄mpyr][OH] solution, through the simple procedure of exposure to CO₂. As it has been discussed (vide supra), the hydroxide anion plays a key role in cellulose dissolution, and thus the conversion of the hydroxide anion is expected to result in cellulose precipitation.

Cellulose Solubility Effect

Initial trials were undertaken to determine the changes in solubility of cellulose in [C₄mpyr][OH]/H₂O upon exposure to CO₂ (and subsequent conversion of the anion of the salt from OH⁻ to HCO₃ ⁻). Upon exposure of [C₄mpyr][OH]/H₂O with 10 wt % dissolved cellulose to CO₂, a white precipitate was gradually formed, which was attributed to cellulose precipitation. Subsequently, the application of this switchable solvent was explored for the formation of cellulose hydrogels. The process for preparing a cellulose hydrogel using cellulose dissolved in [C₄mpyr][OH]/H₂O and CO₂ is presented in Scheme 1, and a plausible gelation mechanism of the cellulose dissolved in [C₄mpyr][OH]/H₂O is illustrated in Figure 4. To compare with cellulose hydrogels and their mechanical properties synthesized in previous studies (vide infra) with 5 wt % cellulose, 5 wt % cellulose solutions with [C₄mpyr][OH]/H₂O were prepared. For the formation of a gel, a Teflon dish was used. The colorless Avicel^® cellulose was employed, which has a lignin content of 0.4 %. ^[38] The small amount of dissolved lignin may have led to the color change to yellow, as can be seen in Scheme 1. As a result of the CO₂ exposure to cellulose solutions, the fluidity of the cellulose solution decreased, leading to gelation. Subsequent washing with water removed the incorporated [C₄mpyr][HCO₃] and also color‐causing components (suggested to be lignin, vide supra).

Scheme 1.

Fabrication process of cellulose hydrogel by using CO₂ gas.

Figure 4.

Plausible gelation mechanism of the cellulose dissolved in [C₄mpyr][OH]/H₂O.

XRD Anyalysis

XRD measurements were conducted to investigate the changes in the crystalline structure of cellulose before and after regeneration. Figure 5 shows the XRD patterns of cellulose before dissolution and after regeneration. Before dissolution, cellulose exhibited the main diffraction peaks at 15.6 and 22.3 °, attributed to the (110) and (020) crystallographic planes, respectively. After regeneration, cellulose showed the main diffraction peaks at 13.5, 20.1, and 22.6 °, attributed to the (1$\bar{1}$ 0), (110), and (020) crystallographic planes, respectively. ^[39]

Figure 5.

XRD patterns of cellulose and regenerated cellulose.

Based on these results, changes in the crystalline structure of cellulose were observed, and the emergence of cellulose II, as is commonly observed upon regeneration, was confirmed. ^[40] The crystallinity index (see Experimental section) of cellulose before dissolution was 59.7 %, whereas after regeneration, it was 50.2 %. These results suggest that the dissolution of cellulose in [C₄mpyr][OH]/H₂O disrupted both intramolecular and intermolecular hydrogen bonds.

Moisture Content

To investigate the water absorption properties of cellulose hydrogels prepared using the process in this study, moisture content measurements were conducted. The results are shown in Figure 6. The moisture content of the hydrogel prepared at a cellulose concentration of 5 wt % was 950.4 %, while that of the 10 wt % hydrogel was 775.9 %. The moisture content decreased with increasing cellulose concentration. This is thought to be due to the fact that as the cellulose concentration increases, the pore size decreases, resulting in smaller spaces for holding water. ^[41] The cellulose hydrogel prepared using chemical crosslinking with epichlorohydrin (ECH) had a water content of 1550 % at 5 wt % and 640 % at 10 wt %. ^[37] The cellulose hydrogels prepared using the process in this experiment are considered to be relatively stable and less affected by cellulose concentration. Additionally, we note that the moisture content of a cellulose hydrogel prepared using a non‐aqueous ionic liquid as a cellulose solvent was 1900 %. ^[38] These results suggest that differences in the preparation process can lead to variations in the water absorption properties of the hydrogels obtained.

Figure 6.

Moisture content ratio of cellulose hydrogels.

Mechanical Property

Having confirmed the production of a cellulose‐based hydrogel using the process developed in this study, compression tests were conducted to evaluate the mechanical strength, and the stress‐strain (S−S) curves, shown in Figure 7. For comparison, Table 1 summarizes the maximum compressive strength and elongation at break of cellulose hydrogels from the literature using microcrystalline cellulose (DP=270), produced at the same cellulose concentration in [C₄mpyr][OH]/H₂O and through chemical cross‐linking with ECH, and also at the same cellulose concentration from a published report using a non‐aqueous ionic liquid as a solvent.[ ⁴² , ⁴³ ]

Figure 7.

Stress‐Strain curve of cellulose hydrogel.

Table 1.

Compression test results of various hydrogels.

Sample	Max compression strength/MPa	Fracture strain/%
[C4mpyr][OH]/H2O+CO2	1.2	~100
[C4mpyr][OH]/H2O+ECH[a]	<0.02	53
[C2mim][(MeO)(H)PO2][b]	5.1	~90

[a] [Ref. [42]] (Fracture strain calculated from the original data). [b] [Ref. [43]] (Fracture strain estimated from the published stress‐strain curve).

According to the S−S curve of the hydrogel produced in this study, a maximum compressive strength of 1.2 MPa and an elongation at a break of approximately 100 % were obtained. Cellulose hydrogels cross‐linked with ECH had a maximum compressive strength of approximately 0.02 MPa and an elongation at break of about 53 %, indicating that the hydrogel obtained in this experiment had higher mechanical strength. Cellulose hydrogels produced using a non‐aqueous ionic liquid as a solvent exhibited a maximum compressive strength of 5.1 MPa, higher than the hydrogel obtained in this experiment, but with an elongation at break of approximately 90 %, which was lower than that obtained in this study. These results suggest that different mechanical strengths of cellulose hydrogels can be obtained depending on the differences in the regeneration process, and thus the process described in this study provides a means to produce hydrogels with a particular set of properties.

The simple gelation procedure is expected to provide a facile means for production of materials with various bulk (and indeed micro‐scale) morphologies. Although outside of the scope of this article, it is expected that a suitable method for regeneration of [C₄mpyr][OH]/H₂O from [C₄mpyr][HCO₃]/H₂O after washing of the hydrogel would provide a means for an efficient process with solvent recycling.

Conclusions

We synthesized [C₄mpyr][OH]/H₂O with a high cellulose solubility and investigated changes in cellulose solubility by introducing CO₂ gas into [C₄mpyr][OH]/H₂O. As the CO₂ flow time increased, the pH of [C₄mpyr][OH]/H₂O decreased from 14 to 9, suggesting its ability to absorb CO₂ gas. From FT‐IR and ¹³C NMR measurements, a sequence of anionic species transformation was observed with increasing CO₂ flow time, proceeding from OH⁻ through a mixed OH⁻/CO₃ ²⁻ to predominantly HCO₃ ⁻. These results indicated the generation of different carbonate species depending on pH value. Moreover, no significant changes were observed in the peak position of the counter‐cation in the ¹³C NMR measurements, suggesting that upon further developing a process to convert the anions back to hydroxide ions, a recyclable cellulose solvent may be achieved.

When CO₂ gas was introduced into the cellulose solution prepared using [C₄mpyr][OH]/H₂O, the cellulose solution was gelated. We established a cellulose regeneration process that is convenient and safe, avoiding the use of flammable and toxic propylene carbonate proposed in earlier work. By applying this process, we envision the development of cellulose products using CO₂ captured from the atmosphere, contributing to further reductions in environmental impact. Mechanical properties of the cellulose hydrogel produced in this experiment were investigated through compression tests, revealing higher strength compared to those created by chemical cross‐linking. These results emphasize the need for further research into materials that can replace non‐degradable, petroleum‐derived substances responsible for environmental pollution.

Experimental Section

Materials

Microcrystalline cellulose Avicel® PH‐101 was purchased from Sigma‐Aldrich and was subjected to vacuum drying at 80 °C for 12 h before use. The degree of polymerization (DP) is reported from 166 to 350. ^[44] N‐Butyl‐N‐methylpyrrolidinium chloride (>99 %) was also purchased from Sigma‐Aldrich. Silver oxide (99 %) and activated carbon (≤100 %) were purchased from FUJIFILM Wako Pure Chemical Corp. and were used as received. CO₂ gas (>99.5 %) was purchased from Tomoe Shokai Co., Ltd. and used as received. Deuterated dimethylsulfoxide (DMSO‐d₆ ) was purchased from Kanto Chemical Co., Inc. and used as received. Tetramethylsilane (TMS) was purchased from FUJIFILM Wako Pure Chemical Corp. and used as received.

Synthesis of Aqueous N‐butyl‐N‐methylpyrrolidinium Hydroxide Solution ([C₄mpyr][OH]/H₂O)

[C₄mpyr][OH]/H₂O was prepared according to the literature. ^[30] N‐Butyl‐N‐methylpyrrolidinium chloride ([C₄mpyr]Cl) (50.0 g, 281 mmol) was dissolved in deionized water (300 mL). Silver oxide (33.4 g, 144 mmol) was added into the solution, and the mixture was stirred at room temperature in the dark for 1 h. The resulting dark brown precipitate was separated by suction filtration, yielding a pale‐yellow aqueous solution. This aqueous solution was decolorized using activated carbon. Subsequently, it was concentrated using an evaporator to generate a concentrated aqueous [C₄mpyr][OH] solution. The water content was determined by Karl Fischer titration. The water content was subsequently adjusted to the desired level by the addition of deionized water to the concentrated aqueous [C₄mpyr][OH] solution. The titration was conducted three times for each sample, and the average value was determined as its moisture content.

Characterization of [C₄mpyr][OH]/H₂O

The characterization of [C₄mpyr][OH]/H₂O was carried out through proton nuclear magnetic resonance spectroscopy (¹H NMR) and Fast Atom Bombardment Mass Spectrometry (FAB‐MS) measurements. ¹H NMR spectra were measured on a Bruker AVANCE III HD NanoBay 400 MHz NMR spectrometer at 25 °C. Double tubes were used with DMSO‐d₆ and TMS as the standard in the inner tube, and the collected samples were added, without dilution, to the outer tube. Mass spectrometry was conducted using a JEOL JMS‐SX102 A instrument. C₄mpyr cations were characterized with positive and negative ion modes, and the parent peak was observed at a mass‐to‐charge ratio of 140.2. ¹H NMR and FAB‐MS data obtained for the synthesized [C₄mpyr][OH]/H₂O were in accordance with literature reports. ^[30]

Water content measurements were performed in triplicate for each sample using Karl Fischer titration at room temperature with the Kyoto Electronics MKH‐710, and the average values were utilized. The titration was adjusted for titration coefficient using KEMAQUA Water Standard 10, and thus the titration performance was verified.

Evaluation of CO₂ Absorption Properties of [C₄mpyr][OH]/H₂O

The experimental procedure is illustrated in Figure S1. Approximately 5 g of [C₄mpyr][OH]/H₂O (50 wt % aqueous solution adjusted for water content) was placed in a three‐neck flask. The contents of the flask were stirred at room temperature. A tube connected one of the flask′s openings to a gas flowmeter, and CO₂ gas (≥99.5 vol %) was introduced into the flask at a rate of 100 mL min⁻¹ (4.46 mmol min⁻¹) for 70 min. A small amount of sample was collected every 5 min. The chemical composition of the samples was investigated through pH, FT‐IR, and ¹³C NMR measurements.

pH Measurements

pH measurements were performed at room temperature on the collected samples using a LAQUAtwin‐pH‐22 compact pH meter. The pH meter was calibrated using pH standard solutions of pH 6.86 (25 °C) and pH 4.01 (25 °C).

FT‐IR Spectrum Measurements

The FT‐IR spectra of the samples were measured at room temperature using the Attenuated Total Reflectance (ATR) method with a diamond crystal. The measurement equipment used for this purpose was the NICOLET 6700 (Thermo Scientific). The measurements were performed with 64 scans and a resolution of 4 cm⁻¹.

¹³C NMR Measurements

The anion composition of the samples was investigated through ¹³C NMR spectroscopy measurements (100 MHz, 1024 scans). The measurements were performed using an AscendTM 400 NMR instrument manufactured by BRUKER, at a temperature of 30 °C. Double tubes were used with DMSO‐d₆ and TMS as the standard in the inner tube, and the collected samples, without dilution, in the outer tube.

X‐ray Diffraction Analysis

X‐ray Diffraction (XRD) measurements were conducted to investigate the crystal structure of microcrystalline cellulose and the cellulose hydrogel prepared. The measurements were carried out at room temperature using monochromatic CuKα radiation (λ=1.5418 Å) on the fully automated multipurpose X‐ray diffractometer, SmartLab, manufactured by Rigaku. The crystallinity index of cellulose I and cellulose II was calculated using equation 1.

$${\displaystyle \mathrm{C}\mathrm{r}\mathrm{I}=100\%\times \frac{I_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}-I_{\mathrm{a}\mathrm{m}}}{I_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}}$$ (1)

Here, I _total represents the intensity of the main peak, and I _am represents the minimum intensity between the main peak and the secondary peak.

Preparation of Cellulose Solutions and Hydrogels

Avicel cellulose, dried on folded filter paper, was weighed at either 5 or 10 wt % (from mass of dissolved cellulose divided by mass of [C₄mpyr][OH]/H₂O) and added to [C₄mpyr][OH]/H₂O adjusted to 50 wt % water content (mass ratios of 10 : 10 : 1 and 5 : 5 : 1 for [C₄mpyr][OH]/H₂O/cellulose with 5 and 10 wt % respectively). Immediately, it was stirred vigorously with a spatula to crush the chunks. Subsequently, stirring was continued at room temperature using a magnetic stirrer. During this stirring process, a significant amount of bubbles were generated in the viscous solution, so degassing was performed using ultrasonic treatment. Afterward, the prepared cellulose solution was transferred to a Teflon dish, and then placed inside a small, sealable plastic pouch to introduce CO₂ gas at a rate of 100 mL min⁻¹.

Moisture Content Ratio of Cellulose Hydrogels

The swelling behavior of the hydrogels was investigated by measuring their moisture content. The fabricated hydrogels were washed with distilled water to remove [C₄mpyr][OH] components, resulting in white hydrogels. The hydrogels were superficially dubbed with filter paper to remove excess surface water, and the weight of the swollen hydrogels (W_wet) was measured. Subsequently, the hydrogels were freeze‐dried to remove moisture, and the dried weight (W_dry) was measured. The moisture content of the hydrogels was calculated using equation 2.

$${\displaystyle \mathrm{M}\mathrm{o}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=100\%\times \frac{W_{wet}-W_{dry}}{W_{dry}}}$$ (2)

Mechanical Properties of Cellulose Hydrogels

To evaluate the mechanical properties of the obtained hydrogel, compression tests were conducted using an Imada compression testing machine (MX2‐500 N). The compression tests were performed at room temperature, with a crosshead speed of 10 mm min⁻¹. The elongation ratio of the hydrogel was determined from the displacement of the crosshead during the test.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

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Supporting Information

Matsui A., Ayu Putri D., Thomas M. L., Takeoka Y., Rikukawa M., Yoshizawa-Fujita M., ChemSusChem 2025, 18, e202401848. 10.1002/cssc.202401848

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.