Chemical Engineering Journal

Ionic liquid-​mediated facile synthesis of porous cellulose/silica aerogel (SA) composites

with improved thermal insulation
--Manuscript Draft--

Manuscript Number: CEJ-D-24-35818

Article Type: Research Paper

Section/Category: Green and Sustainable Science and Technology

Keywords: ionic liquids, silica aerogels, cellulose, porous materials; thermal insulation

Abstract: Silica aerogel (SA) exhibits significant potential as a thermal insulation material, but
major challenges remain in directly dispersing it in polymers meanwhile preserving its
nanoporous structure. This study reports a simple method to stably disperse SA in
cellulose solution using ionic liquid through a mechanical stirring-foaming process,
followed by the regeneration to prepare cellulose/SA composites of aerogels and
aerogel fibers with high thermal insulation. The viscosity of the cellulose solution is the
key factor for the stable dispersion of SA and the preservation of air bubbles. To
achieve aerogels with uniform pore morphology, a bottom-to-top regeneration
technique (BtTR) is utilized during the regeneration process, leading to anti-gravity
diffusion of water from the bottom to the top of the as-foamed cellulose/SA solution.
Unlike traditional top-to-down diffusion, this technique prevents water-induced pressure
that can result collapse of air bubble in the foamed cellulose/SA solution. After freeze-
drying, we produce SA-reinforced thermal insulation cellulose-based aerogels with
uniform pore morphology, low density, and excellent thermal insulation, achieving a
thermal conductivity of 0.034 W m⁻¹ K⁻¹. Additionally, the spinnability of the cellulose-
based solution allows for the production of porous aerogel fibers. This study offers a
new strategy to disperse SA in cellulose assisted by IL, which could be valuable for
inspiring the design and construction of multifunctional aerogel materials for
engineering applications.

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Graphical Abstract

Ionic liquid-mediated facile synthesis of porous cellulose/silica aerogel

(SA) composites with improved thermal insulation

Xinran Liua, Tongping Zhanga, Hongze Xua, Boxiao Lia, Gang Weib, Xiaofang Zhanga*,
Jianming Zhanga*
Highlights (for review)

A simple method to obtain cellulose/SA composites with uniform pore morphology.

SAs were uniformly dispersed in cellulose/SA composites with preserved nanoporous

structure.

Propose the bottom-to-top regeneration technique to avoid collapse of air bubbles.

Foamed cellulose/SA solution exhibits excellent spinnability to fabricate aerogel fibers.

The cellulose raw material used originates from waste pure cotton cloth.
Manuscript Click here to view linked References

1 Ionic liquid-mediated facile synthesis of porous cellulose/silica aerogel (SA)

2 composites with improved thermal insulation

3

4 Xinran Liua, Tongping Zhanga, Hongze Xua, Boxiao Lia, Gang Weib, Xiaofang Zhanga*, Jianming
5 Zhanga*

a
7 Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of

8 Rubber-Plastics, Qingdao University of Science & Technology, Qingdao 266042, China

b
9 College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, PR China

10 *Corresponding author: Jianming Zhang (Email: zjm@qust.edu.cn)

11 *Corresponding author: Xiaofang Zhang (Email: xfzhangyuer@126.com)

12

13

1
14 Abstract
15 Silica aerogel (SA) exhibits significant potential as a thermal insulation material, but major challenges
16 remain in directly dispersing it in polymers meanwhile preserving its nanoporous structure. This study
17 reports a simple method to stably disperse SA in cellulose solution using ionic liquid through a
18 mechanical stirring-foaming process, followed by the regeneration to prepare cellulose/SA composites
19 of aerogels and aerogel fibers with high thermal insulation. The viscosity of the cellulose solution is
20 the key factor for the stable dispersion of SA and the preservation of air bubbles. To achieve aerogels
21 with uniform pore morphology, a bottom-to-top regeneration technique (BtTR) is utilized during the
22 regeneration process, leading to anti-gravity diffusion of water from the bottom to the top of the as-
23 foamed cellulose/SA solution. Unlike traditional top-to-down diffusion, this technique prevents water-
24 induced pressure that can result collapse of air bubble in the foamed cellulose/SA solution. After
25 freeze-drying, we produce SA-reinforced thermal insulation cellulose-based aerogels with uniform
26 pore morphology, low density, and excellent thermal insulation, achieving a thermal conductivity of
27 0.034 W m⁻¹ K⁻¹. Additionally, the spinnability of the cellulose-based solution allows for the
28 production of porous aerogel fibers. This study offers a new strategy to disperse SA in cellulose assisted
29 by IL, which could be valuable for inspiring the design and construction of multifunctional aerogel
30 materials for engineering applications.
31
32 Keywords: ionic liquids, silica aerogels, cellulose, porous materials; thermal insulation
33

2
34 1. Introduction
35 Heat transport and thermal insulation are crucial for managing the thermal performance of
36 electronic devices and controlling the indoor environment in buildings [1]. To achieve energy
37 conservation, significant improvements of insulation are necessary to enhance building energy
38 efficiency. Polymer foams are widely used in construction due to their low density, high mechanical
39 strength, and excellent thermal and sound insulation properties [2]. Commonly used polymer foams
40 include polyurethane (PU) [3], polystyrene (PS) [4], and phenol formaldehyde (PF) [5]. Conventional
41 PU and PF foams rely on fossil fuel-based polyols and phenols, which are costly and environmentally
42 harmful [6]. With increasing environmental concerns, researchers are extensively exploring renewable
43 foam materials.
44 In particular, cellulose is widely used in thermal insulation field due to its abundance, sustainability,
45 and chemical stability [7, 8]. For example, Qi et al. prepared nanocellulose from cotton, wood, bamboo,
46 and straw, which was then used to derive thermal insulation aerogels. They found that the thermal
47 conductivities of the formed aerogels ranged from 0.039 to 0.045 W m⁻¹ K⁻¹ [9]. Additionally, Li et al.
48 prepared all-cellulose gradient concentration sponge-aerogel fibers using microfluidic chips, which
49 significantly reduced the gas conduction within the aerogel. The internal pore size was approximately
50 34 nm, smaller than the average free path of air molecules (about 60 nm), resulting in lower thermal
51 conductivity than air [10].
52 Silica aerogel (SA) is a engineering material with excellent thermal insulation properties, and its
53 incorporation into polymers can significantly enhance the material's thermal insulation performance.
54 There are currently two approaches to introducing SAs into the cellulose skeletons of aerogels. One
55 method is directly incorporating SA powder, while the other involves in-situ generation of SAs within
56 the aerogels. Although most studies focused on in-situ synthesis, this method had clear drawbacks,
57 such as the use of toxic reagents and a complicated drying process. Directly incorporating commercial
58 SAs into cellulose skeletons is a simpler and more effective method. However, the fluidity of SA
59 powder significantly limited its application range, and maintaining the nanopores of SA within
60 composites presents an additional challenge [11-13].
61 Amphiphilic polymers, such as polyvinyl alcohol (PVA), have been reported to stably disperse
62 SAs by utilizing their viscosity and interaction between chains of PVA and SAs, meanwhile preserving
63 the nanopores [14]. Cellulose is an amphiphilic polymer, with its molecules containing numerous
3
64 hydroxyl groups that interact with water molecules via hydrogen bonding, exhibiting strong
65 hydrophilia. The main chain of cellulose consists of glucopyranose rings, composed of C-H bonds,
66 imparting a degree of hydrophobicity. Over the past few decades, various cellulose-dissolving systems
67 have been successfully developed, including N-methyl morpholine-N-oxide hydrate [15], alkali/urea
68 [16], and lithium chloride/N, N-dimethylacetamide [17], etc. However, these solvents presented
69 significant challenges for practical usage, including high toxicity, difficult solvent recovery, and low
70 solubility [18]. Moreover, the nanopores in the hydrophobic SAs would be destroyed by the organic
71 solvent. In contrast, ionic liquids (ILs) are specific salts composed of organic cations and inorganic or
72 organic anions with melting points below 100 °C. They have garnered significant attention due to their
73 low volatility, high thermal stability, and excellent solubility, making them attractive as green,
74 environmentally friendly solvents for dissolving and processing cellulose [19-25]. It has been
75 demonstrated that ILs can effectively dissolve raw cellulose materials to obtain cellulose solution with
76 high concentration, facilitating the fabrication of cellulose products with desirable mechanical
77 properties and functions (e. g., fibers, films, aerogels or foams) [26]. Meanwhile, IL has strong polarity,
78 making it difficult to penetrate hydrophobic SA and disrupt its nanopores. Therefore, based on the
79 amphiphilicity of cellulose and the strong polarity of IL, we hypothesize that SA can disperse in
80 cellulose/IL solutions while preserving its porous structure. Previously, we observed that with the
81 increasing population and continuous economic development, a substantial amount of waste textiles
82 was constantly being generated [27]. Especially, cotton is an abundant textile raw material, but is often
83 discarded as waste [28]. How to achieve sustainable using of cotton in the construction of functional
84 composite biomass materials has great meaning.
85 In this study, we use the IL (1-Allyl-3-methylimidazolium chloride, AMIMCl) as solvent to
86 dissolve recycled waste pure cotton cloth (WPCC) to obtain cellulose-based porous materials (foams
87 & porous cellulose-based fibers), during which mechanical stirring-induced air bubbles are used as
88 templates. To further enhance thermal insulation property of materials, SA is incorporated. The
89 viscosity of the solution stabilizes the dispersion of SA, ensuring uniform distribution within the
90 aerogel and preserving the integrity of the nanopores. To achieve uniform pore morphology in the
91 WPCC/SA aerogel, we innovatively introduce regenerated bath water from the bottom to the top of
92 the foamed cellulose solution. This process forms cellulose-based hydrogels from the bottom to top of
93 the solution, fixing the air bubble templates in place. This method largely avoids traditional top-to-
4
94 down diffusion which often result collapse of air bubble in the foamed cellulose/SA solution. The
95 thermal conductivity of obtained WPCC/SA aerogel is as low as 0.034 W m-1 K-1, at room temperature.
96 Interestingly, the foamed cellulose-based solution could be used to spin. The average pore diameter of
97 the prepared porous cellulose-based fibers is approximately 250 μm. Incorporating air bubble
98 templates and SA results in lightweight, thermally insulating fibers that could be used in functional
99 textiles.
100

101 2. Experimental section

102 2.1 Materials

103 Cotton is the use of post-consumer cotton-clothing. AMIMCl (purity: 96%) was purchased from
104 Macklin. Sodium dodecyl sulfate was purchased from Tianjin Bodi Chemical Co., LTD. Hydrophobic
105 modified SA was provided by IBIH advanced materials Co., Ltd. Corporation. The water used in all
106 experiments was deionized water.
107

108 2.2 Preparation of WPCC/SA aerogels

109 Firstly, WPCC was smashed into tiny fragment, and IL (AMIMCl) was dried by heating. WPCC
110 was then added into the IL according to the ratio of 1 wt%, 1.5 wt%, 2 wt%, and 2.5 wt% of the IL,
111 respectively. The mixture was mixed uniformly under magnetic stirring. Then, the mixture was
112 subjected to high temperature to dissolve WPCC at 110 ℃. When WPCC was fully dissolved, it was
113 taken out to cool down to room temperature. According to the proportion of SA to WPCC was10 wt%,
114 SA was added into the IL. After that, 1 wt% surfactant was added by mass of IL and dissolve in oven
115 at 60 ℃. Secondly, the sample was foamed under magnetic stirring at 500 rpm for 10 min, followed
116 by pouring them into a column bottle whose bottom was cover with a filter (pore size: ~20 μm).
117 Deionized water was as regeneration bath and the water was diffused from bottom to the top. At this
118 point, regenerated cellulose gels was obtained. After the post-treatment process of solvent replacement,
119 the gels were subjected to freeze-drying. Finally, the WPCC/SA-X aerogels (WPCC/SA-X) were
120 obtained, in which X denotes content of the incorporated SA. When the value is zero, WPCC/SA-0 is
121 the WPCC aerogel. The content of WPCC in WPCC/SA-X is 1.5 wt%. And in WPCC-X, the X denotes
122 content of WPCC used in WPCC aerogel. Noteworthily, if the obtained reclaimed cellulose-based bulk

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123 is substandard, it is possible further dissolve it again to prepare approving samples (Figure S1).
124

125 2.3 Preparation of porous WPCC/SA aerogel fibers

126 The preparation of porous WPCC/SA aerogel fibers is following by this procedure. Firstly, 1 wt%
127 aramid nanofiber dimethyl sulfoxide solution was added into the foamed WPCC/SA solution. Then,
128 the foamed mixture was extruded from a syringe needle (diameter: 1.5 mm) into coagulation bath water.
129 After the post-treatment process to remove the redundant IL, the cellulose-based gel fiber was
130 subjected to freeze-drying. Finally, the porous WPCC/SA-10 aerogel fibers with uniform pore
131 morphology were obtained.
132

133 2.4 Characterization techniques

134 In order to investigate the stability of bubbles with different content of waste cotton fibers, using a
135 viscometer (Shanghai Jingtian, SNB-RH) to obtain the viscosity of solutions. Polarizing microscope
136 (POM, Eclipse LV10N POL, Nikon) for observing the morphology of bubbles. For the purpose of
137 investigating the formation of form Ⅱ in cellulose, X-ray diffraction (XRD) patterns were recorded on
138 a diffractometer (Rigaku, ULTIMALV, Cu-kα radiation, λ=0.154 nm) range from 5° to 50° with a scan
139 rate of 5° min-1. All the samples were prepared by freeze drying. The cross-sectional morphology of
140 samples was observed by Scanning electron microscopy (JEOL, JMC-7000) at an accelerating voltage
141 of 10 kV. Specimens were put on the stage of the Vacuum Turbo Evaporator and sputter-coated with
142 gold twice. FT-IR spectra of WPCC/SA aerogels were obtained by a Bruker VERTEX70 spectrometer
143 at the range of 4000 to 500 cm-1 and resolutions of 4 cm-1. Thermogravimetry Analysis (TGA) test was
144 performed by a thermogravimetric analyzer (NETZSCH TG 209F1 Libra) heating from 30-800 ℃
145 with 10 ℃ min-1. The test was carried out in nitrogen atmosphere. The IR images were captured with
146 a Flir One Pro IR camera. (IR). Thermal conductivity was evaluated by a thermal conductivity meter
147 (TPS 2500S, Hot Disk, Switzerland) with the testing temperature range from room temperature to
148 200 ℃. The compressive modulus and compressive strength of the samples were measured by a
149 compression test using an INSTRON 5900 universal testing machine. Before testing, the samples were
150 cut into a cylindrical shape with 32 mm in diameter and 8 mm in thickness.
151

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152 3. Results and discussion

153
154 Figure 1. (A) Schematic synthesis illustration of the WPCC/SA composites with high thermal
155 insulation by the mediation of AMIMCl. (B) POM image of foamed WPCC/SA solution. (C) The photo
156 of the bottom-up regeneration technology. (D) The photograph of WPCC/SA-10 aerogel (diameter: 32
157 mm, height: 15 mm). (E) SEM image of porous WPCC/SA aerogel.
158

159 In this contribution, tough and environmentally friendly reclaimed cellulose-based porous
160 composites were developed for using as thermal insulation materials, taking benefits of the
161 recyclability of WPCC and high porosity of SA. Figure 1 illustrates the preparation of SA-enhanced
162 reclaimed cellulose-based thermal insulation materials (WPCC/SA). The IL (AMIMCl) was chosen as
163 a solvent to fully dissolve WPCC, yielding cellulose molecules (Figure 1A). SA cannot be dissolved
164 in highly polar ionic liquids, allowing the nanopores to be preserved during the compounding process,
165 which enhances the material's thermal insulation. Foamed cellulose solution with a pore size of 50-
166 280 μm was obtained through mechanical foaming, facilitated by the addition of the surfactant SDS
167 (Figure 1B). Water is used as a regeneration bath, diffusing from the bottom to the top to induce
168 gelation of the cellulose chains, thereby stabilizing both the structure and the position of the bubbles
169 (Figure 1C). Finally, porous WPCC/SA aerogels were produced using a freeze-drying method (Figure
170 1D). The resulting WPCC/SA aerogels with hierarchical pore morphology show potential for

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171 applications in thermal insulation.

172
173 Figure 2. (A) The photograph of SA suspending on IL without stirring. (B) The viscosity of foamed
174 WPCC/SA solutions and the inserts on the bottom-right corner is corresponding bubble stability. (C)
175 POM image of foamed WPCC/SA solution. (D) Average pore diameter of foaming solution at different
176 WPCC concentration. (10 wt% SA) (E) The planary concentration of the pores with different amount
177 of WPCC. (F) Diagram of diffusion of the water from the bottom to top of the foamed WPCC/SA
178 solution. (G) Diffusion rate of water from bottom to the top of the foamed WPCC/SA solution. (H)
179 Comparison of hydrogels prepared by top to bottom and bottom to top diffusion methods. (I) Schematic
180 illustration of the mutual diffusion process of AMIMCl in water. (J) Photographs of AMIMCl diffusing
181 into water during regeneration.
182

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183 SA is a highly porous material composed of silica with abundant nanopores. It has been
184 extensively studied for its extremely low density, high porosity, and excellent thermal insulation
185 properties. To preserve the nanopores of SA in the WPCC/SA aerogels, SA was hydrophobically
186 modified by introducing abundant methyl groups [29]. As a result, IL was unable to dissolve SA,
187 indicating that the nanopores of SA were retained (Figure 2A). Optimizing the viscosity of the cellulose
188 solution benefited both the stable dispersion of SA and the stability of the bubble template. Figure 2B
189 shows the viscosity of cellulose solutions with varying amounts of WPCC. As the concentration of
190 WPCC increased from 0.5 to 1.5 wt%, the viscosity of the solution was increased from 7.7 to 46.576
191 Pa·s. Additionally, the stability of the bubbles increased. As shown in the inset of Figure 2B, no
192 significant phase separation was observed in the solution even after being stationary for over 12 hours.
193 However, higher viscosity in the cellulose solution could hinder the introduction of air bubbles during
194 mechanical stirring [30]. Figure 2C shows that the size distribution of bubbles in the foamed cellulose
195 solution rangs from 50 to 280 μm.
196 Additionally, we examined the effect of the WPCC concentration on the bubble diameter and
197 planar density. As shown in Figure 2D and E, as the WPCC concentration increased, the bubble
198 diameters gradually decreased from 46 to 29 μm, while the number of bubbles per mm2 increased from
199 151 to 1207. No significant bubble breakdown or foam shrinkage was observed in the solution with
200 1.5 wt% WPCC, indicating sufficient viscosity to retain the pore structure. Given that a high
201 concentration of WPCC can lead to a high density of the WPCC/SA aerogel, the optimized WPCC
202 concentration was determined to be about 1.5 wt%.
203 In the regeneration process shown in Figure 2F, the WPCC/SA solution was placed on top of a
204 porous filtration cloth, with a water tank positioned underneath. Driven by the concentration gradient,
205 as described by Fick’s law, mutual diffusion occurred: water passed through the filtration cloth and
206 diffused upward from the bottom to the top of the WPCC/SA solution, while AMIMCl diffused in the
207 opposite direction towards the water tank. This mutual diffusion resulted in the gelation of cellulose/SA
208 due to the insolubility of cellulose in water. We named this unconventional technique, bottom-to-top
209 regeneration, as BtTR. To verify this, we monitored the diffusion interface during the BtTR process
210 and measured the diffusion distance of water at fixed intervals (Figure 2G). The diffusion speed was
211 initially fast for the first 5 hours, but gradually slowed down as the concentration gradient of AMIMCl
212 decreased during the continuous diffusion process. Additionally, camera images (Figure 2J) captured
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213 the diffusion of the AMIMCl phase into the water.
214 BtTR successfully mitigated the Ostwald ripening effect, which describes the phenomenon where
215 larger bubbles grow at the expense of smaller ones in a dispersed system, driven by the minimization
216 of total free energy. This effect is undesirable, as it can lead to uncontrollable and unstable morphology,
217 such as a wide distribution of pore sizes, uneven pore density, and collapse of the pore structure. It is
218 a common phenomenon in foaming solutions, often resulting in porous materials with undesirable
219 gradient pore morphology. To avoid this problem, some methods were explored, for example, reducing
220 the free energy of system or enhancing the homogeneity of air bubbles [31]. In this system, bubbles
221 are subjected to different forces. If the diffusion method is applied from top to bottom, the presence of
222 water gravity will exacerbate the uneven distribution (Figure S2 and S3). The resulting material
223 exhibits inhomogeneous pore morphology (Figure S4). We suggest that in the foaming solution,
224 gravity plays a significant role as a key factor influencing the Ostwald ripening effect. The lower
225 bubbles experience greater gravitational force than the upper bubbles, causing them to merge into
226 larger bubbles due to the uneven force distribution.
227 To address this problem, we chose the BtTR technique to minimize the influence of water gravity,
228 allowing the bubbles to retain a uniform structure. In the BtTR technique, the condensed WPCC/SA
229 phase with higher density grows upward due to the water tank being located beneath it. The upward
230 growth of the WPCC/SA phase exerts no external pressure on the liquid phase, resulting in a smooth
231 and stable gel structure (Figure 2H). In contrast, when the positions of the water tank and the
232 WPCC/SA solution was exchanged, defoaming and gel collapse were observed, as shown in Figure
233 2H. Additionally, the resulting product exhibits inhomogeneous pore morphology. The corresponding
234 schematic illustration is shown in Figure 2I, where water diffuses from bottom to top due to the
235 concentration differences. Fortunately, when the coagulation bath water diffusing from bottom to top,
236 it could preferentially fix the air bubbles and avoid the fusion.

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237
238 Figure 3. Structure and composition characterizations of aerogels. (A, B) SEM images of
239 WPCC/SA-10, the insert: the photograph of a WPCC/SA-10. (C) The enlarged SEM image of single
240 cell wall, showing presence of SA in WPCC/SA-10. (D) EDX spectroscopy of WPCC/SA-10. (E) The
241 average pore size of each part of the WPCC/SA-10 and the corresponding SEM images of different
242 sections. (F) Comparison of XRD spectra of WPCC/SA-10 aerogels, WPCC raw material and SA. (G)
243 IR spectra of SA, WPCC raw material, WPCC/SA-10 and WPCC-1.5.
244

245 The inner pore morphology of the WPCC aerogels obtained by freeze-drying was subsequently
246 characterized. As shown in the inset of Figure 3A, the synthesized WPCC/SA-10 sample reveals a
247 smooth, intact appearance without noticeable cracks. Figures 3A and 3B show SEM images of the
248 inner pore morphology of WPCC/SA-10, corresponding to the middle section. The spheroidal pores,
249 originating from the air bubble templates, are dense and uniform, with diameters ranging from 200 to
250 400 μm. A further enlarged SEM image of the pore wall reveals many protrusions of varying sizes, as
251 indicated by the positions circled in red dashed lines (Figure 3C). These bumps are suspected to be
252 agglomerated SAs due to the presence of SAs. To validate this conjecture, the EDS elemental analysis
253 of the pore wall was conducted. As shown in Figure 3D, the red circle highlights a dense distribution
254 of silicon elements, corroborating the hypothesis. This evidence confirms that SA was insoluble in
255 AMIMCl, retaining its shape and becoming encapsulated within WPCC/SA-10 by the WPCC foamed
256 solution due to its desired viscosity.

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257 As mentioned above, the BtTR technique we used in this study can effectively avoid the Ostwald
258 ripening effect and produce a uniform foaming sample, which was further confirmed by the statistical
259 average pore sizes of the top, middle, and bottom layers of the sample. As shown in Figure 3E, the
260 measured pore sizes of the upper, middle, and lower layers in WPCC/SA-10 are 329, 326, and 322 μm,
261 respectively, with only slight, negligible differences. We suggest that the tested sample has good
262 uniformity. It is well known that cellulose I is the natural form of cellulose, while regenerated cellulose
263 mainly exists as cellulose II [32]. We performed XRD characterization of the regenerated WPCC/SA-
264 10, and the result is shown in Figure 3F. The WPCC raw material shows a clear cellulose I
265 characteristic peak in the XRD pattern, and the regenerated WPCC/SA-10 demonstrates characteristic
266 peaks of cellulose II, corresponding to the reported spectrogram [33]. Additionally, the added SA did
267 not alter the crystal structure of the regenerated cellulose. The obtained infrared spectroscopy also
268 agrees well with the XRD data (Figure 3G).

269
270 Figure 4. The mechanical and thermal insulating performance of material. (A) The density of the
271 samples with different WPCC concentration. (B)the corresponding stress-strain curves under different
272 WPCC concentration. (C) The Young’s modulus and maximal strength of the samples (10 wt% SA)
273 with different WPCC concentration. (D) TGA curves and (E) the corresponding DTA curves or WPCC-
274 1.5, WPCC/SA-5 and WPCC/SA-10. (F) Thermal conductivity of WPCC/SA aerogel with different
275 amount of SA (WPCC concentration: 1.5 wt%).

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276

277 Furthermore, the performance of WPCC materials was characterized. As shown in Figure 4A, the
278 density of the WPCC/SA aerogels gradually increased with the WPCC concentration, with the lowest
279 density was observed at 17 mg cm-3. The density of the sample can be adjusted by controlling the
280 content of dissolved WPCC, allowing for customization based on specific requirements. Figures 4B
281 and 4C present the mechanical properties of WPCC aerogels, represented by compressibility and
282 modulus. It can be seen that all compressive stresses and Young’s modulus at the corresponding strain
283 increased with the content of dissolved WPCC, demonstrating the tunable mechanical properties.
284 Specifically, at 20% strain, the WPCC aerogel exhibited a linear elastic region. As the strain increased,
285 the stress entered a plain region, corresponding to the plastic deformation of the spherical pores. Upon
286 further compression, the pores became denser and the bulk density increased, resulting in a further
287 increase in the Young’s modulus [34].
288 To achieve excellent thermal insulation properties, we incorporated SA into the WPCC aerogels.
289 The thermostability of the WPCC/SA aerogels was also characterized. As shown in Figures 4D and
290 4E, the final ash content of WPCC-1.5 approached zero, while that of WPCC/SA-5 and WPCC/SA-10
291 was higher, indicating the presence of incorporated SA. Further analysis indicated that the
292 thermostability of the WPCC aerogels was not improved by the incorporated SA, as it exhibited almost
293 the same initial degradation temperature. Conversely, the maximum degradation temperatures of
294 WPCC/SA-5 and WPCC/SA-10 were lower than that of WPCC-1.5. This may be due to the abundant
295 methyl groups on the chemically modified SA. Fortunately, all the SA-composited WPCC/SA aerogels
296 in our study could withstand temperatures up to 220 °C. The thermal insulation property can be
297 assessed by examining the thermal conductivity values that depicted in Figure 4F. While increasing
298 SA can reduce thermal conductivity, excessive incorporation can lead to agglomeration. The lowest
299 thermal conductivity of SA-composited WPCC aerogels reached 0.034 W m⁻¹ K⁻¹ in WPCC/SA
300 aerogels, demonstrating excellent thermal insulation properties, which could benefit to potential
301 applications.

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302
303 Figure 5. Mechanism of thermal insulating and demonstration of the aerogels. (A) SEM image of
304 WPCC/SA-10. (B) Nitrogen adsorption-desorption isotherms and (C) DFT pore distributions of
305 WPCC/SA-10, WPCC-1.5 and SA. (D) Thermal insulation mechanism of WPCC/SA aerogel. (E) The
306 comparison of the thermal insulation effect between WPCC/SA-10 and other commercially available
307 products. (F) The thermal insulating property of WPCC/SA-10 incorporated with 30 wt% flame
308 retardant ammonium polyphosphate. (G) The temperature variation of the cold side over a 30-minute

309 period for WPCC/SA-10 placed on a 90 °C hot plate. The insert: infrared images at the time intervals

310 of 0 min and 30 min. (H) Effect of thickness on thermal insulating performance of WPCC/SA-10.
311

312 The WPCC/SA-10 exhibits hierarchical pore morphology, including macropores originating from
313 air bubble templates, mesopores resulting from the aggregation of cellulose chains, and micropores
314 formed by the incorporated SAs (Figures 5A, 3C, and 3D). This purposeful design ensures the thermal
315 insulation properties of WPCC/SA-10. The nitrogen adsorption-desorption isotherms and BJH pore
316 distributions of both WPCC-1.5 and WPCC/SA-10 aerogels reveal key structural characteristics. The

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317 specific surface area (SBET) of WPCC-1.5 was approximately 65 m²/g, while that of WPCC/SA-10
318 increased to about 100 m²/g (SA's SBET was 553 m²/g) (Figure 5B). This indicates that the pores of
319 SA were retained in WPCC/SA-10, which is also supported by the DFT pore distribution analysis
320 (Figure 5C). The WPCC/SA-10 aerogels had a smaller average pore diameter than the WPCC-1.5
321 aerogels. We suggest that the synergistic effects contributed to their thermal insulation and flame-
322 retardant properties.
323 Considering the impact of cell structure and SA on the thermal insulation properties of the
324 aerogels, we measured the thermal conductivity and insulation performance of WPCC/SA-10. It has
325 been reported that in porous materials, the heat transfer due to fluid convection can be largely neglected
326 when the pore size was less than 4 mm. Since the internal pores of this material are primarily on the
327 micron scale, the influence of convection is disregarded in its thermal insulation mechanism. Therefore,
328 the conduction and radiation are identified as the two primary modes of thermal insulation for this
329 material. These results indicate that the aforementioned factors significantly affected the thermal
330 insulation properties of WPCC/SA-10. Figure 5D illustrates a probable thermal insulation mechanism
331 of the WPCC/SA aerogels. The assembled hierarchical structure and the presence of SA simultaneously
332 reduced both the overall density and solid heat conduction. Meanwhile, the pores reduced the direct
333 contact area within the solid skeleton, thereby minimizing the thermal bridge effect, which is the rapid
334 heat conduction through the solid skeleton. This reduction improved the overall thermal insulation
335 performance. Additionally, the porous structure of the aerogels caused heat radiation to be repeatedly
336 reflected and scattered within the material, complicating the transfer path and reducing the efficiency
337 of heat radiation.
338 To demonstrate the superior thermal insulation properties of WPCC/SA-10, we compared it with
339 other commercial materials, including polyethylene foam, polystyrene foam, melamine foam, and
340 cotton bulk. As shown in Figure 5E, under identical heating conditions, the temperature on the cold
341 side of WPCC/SA-10 was significantly lower than that of the other four materials. In Figure 5F, we
342 tested the flame retardancy of WPCC/SA-10. To enhance this property, 30 wt% flame retardant
343 (ammonium polyphosphate) was incorporated via a simple post-treatment by immersing the sample in
344 the solution. When the flame was in contact with the aerogel for 10 seconds (top images in Figure 5F),
345 the sample turned black but did not self-ignite. When the flame was removed, the sample extinguished
346 quickly, within 100 ms, indicating that the modified WPCC/SA-10 (with flame retardant) had excellent
15
347 flame-retardant properties. Additionally, we investigated the impact of the WPCC/SA-10 thickness on
348 its thermal insulation properties and recorded infrared images using an infrared camera. As shown in
349 Figure 5G, the thickness had a significant influence on the thermal insulation effect of the aerogel.
350 After heating at 90 °C for 30 min, the surface temperature of the 1 cm thick aerogel was about 53 °C.
351 When the thickness was increased to 1.5 cm, the surface temperature dropped to 37 °C. We suggest
352 that as the thickness increased, the internal cell structure became denser, lengthening the path of heat
353 convection, which improved thermal performance. Therefore, customized thermal insulation can be
354 achieved by adjusting the thickness. Furthermore, we monitored the surface temperature of 1.5 cm
355 thick WPCC/SA-10 aerogels (Figure 5H). When heated on a hot plate at 90 °C for 30 min, the surface
356 temperature was still stabilized at 37 °C. Over time, the temperature fluctuated slightly without
357 significant change.

358
359 Figure 6. Preparation of porous WPCC/SA fibers. (A) Schematic illustration of preparation of
360 porous WPCC/SA fiber. (B) Photographs of large-scale preparation of porous WPCC/SA-10 fiber (left),
361 and the TPU-encapsulated fiber with diameter of 0.8 mm hanging 200 g weights. (C and D) SEM
362 images of porous bare WPCC/SA-10 fibers without TPU shell. (E-G) SEM images of porous

16
363 WPCC/SA-10 fibers with TPU shell. (G) The enlarged SEM images TPU encapsulated WPCC/SA-10
364 fibers. (H) The thermal insulation effect of bare porous WPCC/SA-10 fiber after being placed under a
365 hot plate at different temperature for 30 mins. (I) The temperature difference of fibers with and without
366 TPU shell at different temperature. (J) Stress-strain curves of TPU-encapsulated and bare WPCC/SA-
367 10 aerogel fibers.
368

369 The foamed WPCC/SA solution can also be used to prepare thermal insulation aerogel fibers
370 through wet spinning, and a schematic illustration is shown in Figure 6A. The foamed WPCC/SA
371 solution was extruded into a coagulation bath, where water was used to obtain regenerated cellulose
372 hydrogel fiber. Subsequently, the aerogel fibers were prepared by freeze-drying. To improve the tensile
373 strength of porous WPCC/SA fibers, 1 wt% aramid fibers were added. To improve their practicality
374 [35], various efforts have been utilized [36, 37].
375 In this study, a core-shell structure was prepared with porous WPCC/SA-10 fiber as the core layer
376 and TPU as the shell layer, to further enhance its mechanical properties [38]. As shown in Figure 6B,
377 a fiber with a diameter of 800 μm, wrapped in a TPU layer of about 30 μm, could lift a weight of 200
378 g. The pore structure inside the fibers was also investigated by SEM, as shown in Figures 6C to 6G. It
379 can be found that the bare WPCC/SA-10 fibers exhibit uniform spherical pores with sizes consistent
380 with those in the foamed WPCC/SA solution (Figure 2C). However, they are smaller than those in the
381 aerogel bulk mentioned above. This is ascribed to the smaller fiber diameter, faster regeneration rate,
382 and better retention of the air bubble template structure. The SEM images of the porous WPCC/SA-
383 10 fiber coated with a TPU layer are shown in Figures 6F and 6G (sample preparation: hydrogel was
384 slowly frozen, followed by freeze-drying). The pores remained intact without collapse, and the TPU
385 layer, about 30 μm thick, could be seen. The thickness of the TPU layer could be adjusted according
386 to different requirements by varying the concentration of TPU or the impregnation time.
387 To explore their application as thermal insulation, we collected a series of infrared images of
388 porous WPCC/SA-10 fibers at different temperature stages, as shown in Figure 6H. The images were
389 taken when the surface temperature was stable, with the room temperature maintained at around 20 °C.
390 The surface temperatures of porous WPCC/SA-10 fibers were lower than that of the hot plate,
391 indicating good thermal insulation properties. We then compared the corresponding temperature
392 difference (∆T) between the hot stage and the fiber surface (Figure 6I). The thermal insulation
17
393 performance of the porous fiber with a TPU layer (thickness: 30 μm) was slightly lower than that of
394 the fiber without the TPU layer, due to its intrinsic thermal conductivity. However, the encapsulation
395 of TPU could greatly enhance the practical application of porous WPCC/SA-10 fibers. To demonstrate
396 their practical application, we wove the porous fiber into fabric (Figure S5) and found that the ∆T of
397 the single fiber was lower than that of the textile because the thinner fiber was more easily affected by
398 the surrounding environment [38]. The advantage of aerogel encapsulation is that it combines the
399 tensile properties of the shell material with the low thermal conductivity of the aerogel core. As shown
400 in Figure 6G, when the aerogel fibers were encapsulated by the TPU shell, their tensile strength and
401 elongation at break significantly improved, from 1.77 to 6.09 MPa and from 3.7% to 18.5%,
402 respectively.
403

404 4. Conclusion
405 In summary, we utilized the viscosity of the solution, formed by dissolving cellulose in IL, to
406 stably disperse SA. This ensured that SA was evenly distributed throughout the aerogel, effectively
407 enhancing its thermal insulation properties. We developed the BtTR technique to fabricate cellulose-
408 based porous materials using reclaimed cotton cloth, where the gelation began at the bottom of the
409 foamed solution and progressed upward. This technique successfully mitigated the Ostwald ripening
410 effect during the preparation of WPCC/SA aerogels. By manipulating the viscosity of the solution,
411 BtTR achieved a uniform and integrated morphology without apparent gradient in pore diameter along
412 the aerogel. The resulting WPCC/SA aerogel exhibited low thermal conductivity of 0.034 W m⁻¹ K⁻¹,
413 attributed to its low density and hierarchical pore morphology. Incorporating 1 wt% aramid nanofiber
414 into the foamed WPCC/SA IL solution produced porous cellulose-based aerogel fibers with stable
415 mechanical properties. To enhance their practical applications, the mechanical properties of these
416 fibers were further improved by coating them with TPU, resulting in excellent thermal insulation
417 properties. This study presents a new strategy for dispersing SA by utilizing the viscosity of cellulose-
418 ionic liquid solutions, which could inspire the design and fabrication of multifunctional aerogel
419 materials.

420 Supporting Information

421 Ionic liquid process reclaimed cellulose-based aerogel to fabricate ideal cellulose-based. Digital

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422 picture of a cellulose-based gel obtained by regeneration method of from top to bottom of the foamed
423 WPCC/SA solution and the corresponding SEM images. Digital picture of a fabric waved by porous
424 WPCC/SA-10 and its thermal insulation property.

425  ACKNOWLEDGEMENTS

426 Shandong Provincial Natural Science Foundation (ZR2021ME062) are kindly acknowledged for
427 financial support.

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