Methods for coating cellulose wet aerogels with lignin and cellulose aerogels coated with lignin

Abstract

Method for coating cellulose wet aerogels with lignin by (a) Dissolving lignin in a basic aqueous solution, wherein the concentration of lignin in the aqueous solution is 0.1 wt% or more and the pH of the solution is in the range of pH >7 to 14, (b) Neutralizing the basic aqueous solution with organic acids or mineral acids, (c) Adding cellulose moisturizing gels, (d) then exchange of the solvent and (e) Drying with supercritical CO2.

Description

[0001] The present invention relates to a method for coating the surface of cellulose wet aerogels with lignin and the cellulose aerogels obtained by this method.

[0002] Cellulose is the most abundant polysaccharide on Earth and is known for its chemical and thermal stability, low cost, and renewability (Wang, S., Lu, A., & Zhang, L., Recent advances in regenerated cellulose materials. Progress in Polymer Science, 2016, 53: pp. 169–206; Wong, LC, Leh, CP, & Goh, CF, Designing cellulose hydrogels from non-woody biomass. Carbohydr Polym, 2021, 15; 264: 118036). This polymer can be used for the production of aerogels. The term "aerogel" was first coined by Kistler in 1931 and describes a solid gel in which the liquid medium has been replaced by air, with the solid porous network shrinking slightly (Kistler, SS, Coherent Expanded Aerogels and Jellies. Nature, 1931, 3211(127): p. 741). Cellulose aerogels are known for their low density, high porosity, large specific surface area, and low thermal conductivity (Budtova, T., Lokki, T., Malakooti, ​​S., Rege, A., Lu, H., Milow, B.)., Vapaavuori, J., & Livod, S.L., Acoustic Properties of Aerogels: Current Status and Prospects. Advanced Engineering Materials, 2022, 25(6): 2201137; Cai, J., Kimura, S., Wada, M., Kuga, S., & Zhang, L., Cellulose aerogels from aqueous alkali hydroxide-urea solution. ChemSusChem. 2008; 1(1-2): 149-54). Diese Stoffe können unter Anderem als Katalysatoren, Adsorbentien und thermische Isolatoren verwendet werden (Nguyen, S.T., Feng, J., Ng, S.K., Wong, J.P.W., Tan, V.B.C & Duong, H.M., Advanced thermal insulation and absorption properties of recycled cellulose aerogels. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2014, 445: S. 128-134; Han, Y., Zhang, X., Wu, X & Lu, C., Flame Retardant, Heat Insulating Cellulose Aerogels from Waste Cotton Fabrics by in Situ Formation of Magnesium Hydroxide Nanoparticles in Cellulose Gel Nanostructures. ACS Sustainable Chemistry & Engineering, 2015, 3(8): S. 1853-1859).Cellulose aerogels have similar insulating properties to current market alternatives but are safe and renewable. However, they are still very expensive, and their hydrophilicity and flammability prevent their use as thermal insulators. Various strategies can be applied to cellulose materials to improve their hydrophobicity and fire resistance. To enhance the fire resistance of these materials, a protective coating or filler can be added. These can work, among other things, by releasing inert products, by forming radical scavenger compounds that inhibit the combustion reaction, or by forming a protective layer. To promote these mechanisms, the addition of silane groups (silanization) is recommended (Dang, YT, et al., Green fabrication of bio-based aerogels from coconut fibers for wastewater treatment. Journal of Porous Materials, 2022, 29(4): pp. 1265-1278; Setyawan, H., et al.)., Fabrication of Hydrophobic Cellulose Aerogels from Renewable Biomass Coir Fibers for Oil Spillage Clean-Up. Journal of Polymers and the Environment, 2022, 30(12): pp. 5228-5238; Yue, X., et al., Superhydrophobic wastepaper-based aerogel as a thermal insulating cooler for building. Energy, 2022, 245), of carbon chains (esterification, graft polymerization) (Jadhav, PS, et al., Biogenic Straw Aerogel Thermal Insulation Materials. Advanced Engineering Materials, 2023, 25(13)) or the addition of phosphorus- or boron-based structural units to the surface of the cellulose are common alternatives (Sun, J., et al., Thermal-insulating, flame-retardant and mechanically resistant aerogel based on bio-inspired tubular cellulose. Composites Part B: Engineering, 2021, 220). By protecting the oxygen groups in the cellulose, these methods further improve its resistance to moisture and burning (Azman Mohammad Taib, MN, et al.Recent progress in cellulose-based composites towards flame retardancy applications. Polymer, 2022, 244; Liyanage, S., et al., Production and Surface Modification of Cellulose Bioproducts. Polymers (Basel), 2021, 13(19)). In recent years, the use of bio-based, non-toxic alternatives such as tannic acid, phytic acid, and isosorbide has been investigated more closely. Lignin is an interesting alternative because it is abundant and its phenolic structure allows the formation of a carbon layer that can protect the more flammable cellulose (up to 41% combustion residue) (Liang, X., et al., Thermal Kinetics of a Lignin-Based Flame Retardant. Polymers (Basel), 2020, 12(9)). Its antioxidant properties appear to suggest resistance to the radical reactions of combustion. The modification of lignin with phosphorus-nitrogen or silicon oxide groups further enhances its natural fire and moisture resistance (Hobbs, CE)., Recent Advances in Bio-Based Flame Retardant Additives for Synthetic Polymeric Materials. Polymers (Basel), 2019, 11(2); Solihat, NN, et al., Recent Developments in Flame-Retardant Lignin-Based Biocomposite: Manufacturing, and characterization. Journal of Polymers and the Environment, 2022, 30(11): pp. 4517-4537; Costes, L., et al., Phosphorus and nitrogen derivatization as efficient route for improvement of lignin flame retardant action in PLA. European Polymer Journal, 2016, 84: pp. 652-667). The interaction of lignin with cellulose and with modifying agents is hindered by its complex, irregular chemical structure, which is influenced by several factors, including the source of the biomass and the extraction process of lignin (Zhao, C., et al., Revealing Structural Differences between Alkaline and Kraft Lignins by HSQC NMR. Industrial & Engineering Chemistry Research, 2019, 58(14): pp. 5707-5714).Lignin extracted using sulfur processes (kraft and sulfite lignin) contains up to 3% and 8% wt.% sulfur, respectively, in the final product. These materials are not pure and exhibit higher molecular weights and OH groups than lignin produced by sulfur-free processes (soda and organosolv lignin). Costes et al. confirmed that organosolv lignin is less thermally stable than kraft lignin due to its relative content of carboxyl and phenol groups. While sulfur-containing processes are used for hardwood and softwood, sulfur-free processes are employed for agricultural residues. The lignin obtained from the latter process is more hydrophobic than that obtained from the former. Due to its hydrophobicity, lignin can only be dissolved in selected organic solvents or basic aqueous solutions. The solubility of lignin varies considerably depending on its relative chemical composition. Fitigău et al.They concluded that Kraftlignin has a maximum solubility of 50 vol% in an acetone-water mixture, while Sodalignin has a solubility of 90% (Fitigău, IF, F. Peter and CG Boeriu, Structural Analysis of Lignins from Different Sources. International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering, 2013, 7(4)). The dissolution of the commercially widely used kraft lignin in ethanol-water mixtures, alkaline water, and other solvents has been particularly studied, as has its comparison with other lignin classes (Goldmann, WM, et al., Solubility and fractionation of Indulin AT kraft lignin in ethanol-water media. Separation and Purification Technology, 2019, 209: pp. 826-832; Melro, E., et al., Dissolution of kraft lignin in alkaline solutions. Int J Biol Macromol, 2020, 148: pp. 688-695; Sameni, J., S. Krigstin and M. Sain, Solubility of Lignin and Acetylated Lignin in Organic Solvents. BioResources, 2017, 12(1): p.1548-1565). The conditions for lignin precipitation in basic media and the relationships between the pKa of its phenolic groups, the pH, the temperature, and the ionic strength of the solution are still not well understood. In general, at higher salt concentrations, lignin is converted into larger particles and conglomerates, which precipitate at higher pH values. This is due to the electrostatic interactions between salt ions and dissociated phenol and carboxyl groups of lignin, which promote lignin precipitation (Norgren, M. and B. Lindström, Dissociation of Phenolic Groups in Kraft Lignin at Elevated Temperatures. Holzforschung, 2000, 54(5): pp. 519-527; Lindström, T., The colloidal behavior of kraft lignin Part II. Coagulation of kraft lignin sols in the presence of simple and complex metal ions. Colloid & Polymer Sci, 1980, 258: pp. 168-173; Sewring, T., Precipitation of Kraft Lignin from Aqueous Solutions, in Department of Chemistry and Chemical Engineering. 2019, CHALMERS UNIVERSITY OF TECHNOLOGY). These mechanisms are important for quantifying how much lignin can be dissolved in a basic solution and remains dissolved after neutralization. Furthermore, unmodified lignin agglomerates may not interact strongly enough with a matrix if the matrix is ​​hydrophilic and no chemical bond is formed. This poor interfacial adhesion can lead to leaching and a reduction in the mechanical strength of the matrix (Cayla, A., et al., PLA with Intumescent System Containing Lignin and Ammonium Polyphosphate for Flame Retardant Textile. Polymers (Basel), 2016, 8(9); Javed, A., et al., Lignin-Containing Coatings for Packaging Materials-Pilot Trials. Polymers (Basel), 2021, 13(10).The interaction of lignin with cellulose fibers shows that lignin can adsorb onto cellulose (especially in complexation with CaCl2) as conglomerates, but most of it is washed away after washing with water (Maximova, N., et al., Lignin adsorption on cellulose fibre surfaces: Effect on surface chemistry, surface morphology and paper strength. Cellulose, 2001, 8: pp. 113-125; Koljonen, K., et al., Precipitation of lignin and extractives on kraft pulp: effect on surface chemistry, surface morphology and paper strength. Cellulose, 2004, 11(2): pp. 209-224). Computer-aided studies show that the adsorbed lignin interacts with the OH groups of cellulose via electrostatic dipole-dipole interactions through its OH groups, while its phenol rings tend to run parallel to the cellulose chains (Houtman, CJ and RH Atalla, Cellulose-Lignin Interactions (A Computational Study). Plant Physiol, 1995, 107(3): pp. 977-984).In the synthesis of cellulose aerogels, the influence of lignin on the cellulose solution and the cellulose aerogel body was investigated. Costa et al. found that the dissolution of cellulose in NaOH media was promoted by the amphiphilic properties of kraft lignin, which acted like urea (Costa, C., et al., Lignin enhances cellulose dissolution in cold alkali. Carbohydr Polym, 2021, 274: p. 118661; Sescousse, R., A. Smacchia and T. Budtova, Influence of lignin on cellulose-NaOH-water mixtures properties and on aerocellulose morphology. Cellulose, 2010, 17(6): pp. 1137-1146). On the other hand, Sescousse et al. The opposite trend was observed when organosolv lignin was used, while lignin leaching was observed during coagulation (more leaching and higher pore concentration were observed with decreasing acid concentration in the regeneration bath) (Sescousse, R., A. Smacchia and T.Budtova, Influence of lignin on cellulose-NaOH-water mixtures properties and on aerocellulose morphology. Cellulose, 2010, 17(6): pp. 1137-1146). Similar observations have been made by other researchers during the formation of lignin-cellulose composites in monolithic or bead form (Gabov, K., et al., Preparation, characterization and antimicrobial application of hybrid cellulose-lignin beads. Cellulose, 2016, 24(2): pp. 641-658). Aerogels containing lignin become more hydrophobic and exhibit lower thermal conductivity, but lignin leaching and the formation of larger pores reduce the specific surface area of ​​the aerogels (Budtova, T., et al., Biorefinery Approach for Aerogels. Polymers (Basel), 2020, 12(12); Ciolacu, D., et al., MORPHOLOGICAL AND SURFACE ASPECTS OF CELLULOSE-LIGNIN HYDROGELS. Cellulose Chem. Technol, 2013, 47(5-6): pp. 377-386).

[0003] US 2015 / 0368441 A1 discloses an oleophilic and hydrophilic nanocellulose material. The material is coated with lignin. WO 2015 / 126583 A1 discloses cellulose fibers coated with lignin. However, cellulose aerogels and methods for their production are not disclosed therein.

[0004] The state of the art in the synthesis of moisture-resistant cellulose aerogels relies heavily on the use of chemical agents to modify the chemical structure of the cellulose. If lignin is mixed into the cellulose solution without a crosslinking reaction, it is present in the internal structure of the cellulose wet gel and is washed out during the processing steps. Otherwise, toxic crosslinking agents such as epichlorohydrin must be used.

[0005] The object of the invention is to provide a method for effectively coating the surface of cellulose aerogels with lignin and minimizing the influence of water on their internal structure. Surprisingly, it was found that this is possible through a stepwise process that includes a neutralization step prior to the actual coating.

[0006] The aforementioned goal is therefore achieved by a process for coating cellulose wet aerogels with lignin by (a) Dissolving lignin in a basic aqueous solution, wherein the concentration of lignin in the aqueous solution is 0.1 wt% or more and the pH of the solution is in the range of pH >7 to 14, (b) Neutralizing the basic aqueous solution with organic acids or mineral acids, (c) Adding cellulose moisturizing gels, (d) then exchange of the solvent and (e) Drying with supercritical CO2. Accordingly, the first embodiment of the invention comprises a method for coating cellulose wet gels with lignin by first dissolving lignin in a basic aqueous solution, then neutralizing the same and subsequently adding cellulose wet gels in the aforementioned order.

[0007] After solvent exchange and the usual conversion to aerogels by supercritical CO2 drying, a thin lignin layer is present on the surface of the aerogel. The method of the present application makes it possible to cover essentially the entire surface of the aerogel. This layer reduces the amount of water absorbed in aerogels. The coating requires only contact between the cellulose wet gel and the lignin dissolved in water, without any additional reactants or chemical reactions. An aqueous solution according to the present invention consists essentially of water and salts before and after neutralization. The salts are formed by the base added to the water to produce a basic aqueous solution and the acid chemical used for neutralization. For example, if KOH is used to obtain the basic aqueous solution and HCl for neutralization, KCl would be the salt.Depending on the basic and acidic chemicals used, other salts are also obvious to the expert.

[0008] For the purposes of this invention, a basic aqueous solution is understood to have a pH value in the range of pH > 7 to 14. Preferably, the pH value is 13.5 or less, particularly preferably 9 to 13. This is the value at which the lignin dissolves best in the aqueous solution and no precipitation occurs during neutralization.

[0009] Neutralization can be carried out with all known acid components, such as organic acids (acetic acid, lactic acid, etc.) or mineral acids. A preferred mineral acid is HCl.

[0010] The process described here contributes to coating only the surface of the cellulose aerogel with lignin, thereby preserving the aerogel's structure and imparting moisture resistance. The coating occurs through adsorption, minimizing the use of reactants and waste production. According to the present invention, after preparing a neutral lignin solution in water and adding moist cellulose gels to this solution, lignin adheres to the surface of the cellulose gel solely by adsorption. After conventional solvent exchange and shrinkage-free drying, a lignin-coated cellulose aerogel can be obtained. The lignin remains adhered to the surface of the aerogel.While lignin has previously been used in combination with cellulose to produce aerogels, a feature of this invention is that it is applied as a coating agent only on the surface and only by adsorption to achieve moisture resistance.

[0011] To coat the cellulose aerogels, these particles come into contact with a neutralized lignin solution in the wet gel stage. This is possible by maintaining a simple, environmentally friendly, and cost-effective method for producing lignin and cellulose solutions with viscoelastic properties, which allows the creation of wet gel spheres.

[0012] The following steps can be performed to dissolve lignin.

[0013] Step 1: A 0.2 M sodium hydroxide solution is prepared, and lignin is added until a concentration of 2 wt% is reached. The solution is allowed to stand for 24 hours to ensure complete dissolution. Accordingly, a sodium hydroxide solution is used in the first step of the invention. Of course, different bases can be used.

[0014] Step 2: After 24 hours, HCl is added until neutralization is achieved, while maintaining the solution state of the lignin. Of course, different acids can be used.

[0015] Step 3: Moist gel cellulose beads or other structures are added to the neutralized lignin solution, and it is left to stand for 24 hours.

[0016] Step 4: The coated gels are then preferably placed in an acetone bath, where a solvent exchange is carried out before CO2 supercritical drying. For the purposes of this application, the solvent is defined as the basic aqueous solution, i.e., water plus any additives, such as a base to adjust the required pH value.

[0017] Preferably, lignin with low or no sulfur content, such as sodalignin, should be used.

[0018] Preferably, the ratio of lignin to wet gel mass should not exceed 1:5. Dilution of the lignin solution with water to achieve lower ratios is possible.

[0019] Regeneration, neutralization and solvent exchange are preferably carried out at room temperature (20°C) and normal pressure (1 bar).

[0020] Preferably, at least a three-stage solvent exchange should be carried out using only acetone to effectively remove water and prevent cellulose leakage. Step 1: A cellulose solution is prepared by dissolving cellulose in a water:NaOH:urea mixture (81:7:12 wt%). The solution can be stored for extended periods at temperatures below -12°C. For improved process stability, the ideal mass fraction of cellulose is 6-7 wt%. Step 2: The solution is thawed and stirred before being extruded through a drip system into a regeneration bath. In this phase, the spinning solution droplets regenerate and form cellulose wet gel beads. The regeneration bath is an aqueous acid solution. Step 3: The moist gel beads are washed with distilled water until neutralization is achieved, and then with distilled water to remove salts. The beads can be placed in the lignin bath at this stage or even midway through the solvent exchange phase.

[0021] The dissolution of the cellulose should preferably take place in a water:NaOH system. Other additives, such as urea, may also be added. In a preferred embodiment, the concentration of urea is 10 to 15 wt.%, particularly 12 to 13 wt.%, based on the total weight of the solution (i.e., the water:NaOH system).

[0022] Preferably, the regeneration bath is an aqueous solution of an inorganic (sulfuric acid) or organic (e.g., acetic acid, lactic acid) acid with a concentration of up to 2 M. Concentration and drop parameters such as drop height, nozzle diameter, jet flow and others must be adjusted to ensure the generation of a stable jet that can be converted into droplets.

[0023] The following paragraphs refer to the production of cellulose moist gels, which is known in principle to those skilled in the art: Preferably, the solvent for supercritical CO2 drying is replaced by acetone. The solvent chosen for drying is the same one used for the solvent exchange baths. Preferably the nozzle diameter is between 400 µm and 1 mm, the drop height is between 60 cm and 130 cm, and the jet flow rate is between 0.5 g / s and 2 g / s. Regeneration, neutralization, and solvent exchange are preferably carried out at room temperature and atmospheric pressure. The volume ratio of bath to cellulose solution should preferably be greater than 5:1 to efficiently produce spherical beads. Fig. Figure 1 schematically shows the coating of cellulose wet gels with lignin and their conversion into aerogels.

[0024] This process is based on the dissolution of cellulose in a spinning solution and its regeneration, preferably as wet gel beads, on the contact of these beads with a neutralized lignin solution, whereby lignin is deposited on their surface, and it is based on the conversion of these beads into aerogels, whereby their internal structure and moisture resistance are retained.

[0025] The crystallinity of cellulose and the strong network of hydrogen bonds between the cellulose fibers hinder its dissolution. Using a water-NaOH mixture, with or without additives, can disrupt this hydrogen bond network and effectively dissolve cellulose at low temperatures. Contact with a non-solvent, such as an acidic solution, disrupts the interaction between the cellulose and its solvent. The rapid diffusion of non-solvent molecules into the interior of the cellulose droplet and of solvent molecules into the regeneration bath leads to the reorganization of the cellulose fibers through hydrogen bonding into a porous, three-dimensional network. This forms a wet gel cellulose globule. Neutralization is necessary to adjust the pH and remove all salts and other components from the solution medium from the wet gel globules.

[0026] For the lignin solution to work, a high concentration of perfectly dissolved lignin is important, but not so high that neutralization is impossible. The solution conditions depend on the pH of the basic aqueous solution. At a pH of approximately 13, the lignin concentration is about 2% to 3% by weight. If the concentration is too high, precipitation will occur before neutralization. If the concentration is less than 0.1% by weight, the coating is less effective, but still possible.

[0027] The salts formed during neutralization of the solution form complexes with lignin and create large agglomerates that precipitate. The concentration determined for this invention contained the largest amount of lignin that could be neutralized while remaining dissolved in colloidal dimensions. Lower concentrations can be prepared directly, or the wet cellulose gel globules can be added to the lignin bath by dilution after neutralization. Due to the formation of hydrogen bonds between the polar OH functional groups of lignin and cellulose, as well as electrostatic interactions between the aromatic rings of lignin and the pyranose ring of cellulose (both hydrophobic), lignin is adsorbed onto the surface of the wet cellulose gels. This adsorption leads to a competition between the stability of the lignin dissolved in water and its stability when adsorbed onto the wet gel.The lignin particles form a thin layer covering the spheres. Although this distribution is not homogeneous and some areas are visibly uncovered, the majority of the spheres' surface is indeed covered with lignin, while the porous structure inside remains intact. After adsorption, solvent exchange occurs exclusively with acetone, in which lignin is insoluble. This minimizes lignin leaching from the surface. Due to the strength of the hydrophobic interaction and the hydrogen bonding between the two biopolymers, the coating survives the drying phase and remains adhered to the cellulose aerogel spheres even when the samples are scratched.

[0028] Compared to the prior art, the advantageous effects of the present invention are: This invention combines classical methods for dissolving, regenerating, and converting cellulose into aerogels with expertise in lignin dissolution and adsorption. It optimizes conditions for efficient lignin dissolution and adsorption onto cellulose wet gels, minimizing leaching. This allows for the production of lignin-coated cellulose aerogels with enhanced water resistance, eliminating the need for chemical reactions or functionalization processes compared to methods described in the literature. This invention bridges the gap between creating water-resistant cellulose aerogels using a different bio-based polymer without additional reactants or solvents, as it involves the scalable formation and coating of the cellulose wet gel.

[0029] The produced wet gel spheres, after coating and drying, yield aerogels with large specific surface areas (> 100 m²). 2 / g), porous structures, and higher resistance to water absorption. Due to these properties and the high surface area to volume ratio of these spheres, they are particularly interesting for thermal insulation, filtration, and adsorption applications.

[0030] The raw materials used for dissolving and regenerating cellulose, as well as for its neutralization and solvent exchange, are readily available, cost-effective, and easy to use. The developed methods can be scaled up, and all resulting residues can be recycled. The cellulose and lignin used are renewable materials that can be obtained from bio-based waste, in line with a bio-based circular economy. Utilizing these residues increases the economic, environmental, and social benefits of this invention.

[0031] Further data can be observed below. Fig. Figure 2 shows lignin-coated cellulose aerogel spheres in varying lignin:cellulose wet gels. At higher concentrations, the initially white spheres turn browner due to the increased lignin content on their surface. Brushing the dried samples released neither powder nor dust. This indicates that the hydrogen bond between lignin and cellulose is strong enough to withstand mechanical stress. Fig. Figure 2 shows lignin-coated cellulose aerogel spheres with different lignin:cellulose wet gel ratios (L:C).

[0032] After the synthesis of these samples, their water absorption capacity was measured to understand the effectiveness of the coating method. Fig. These data can be observed in step 3. It should be noted that with the same lignin solution volume, it was not possible to achieve lignin-to-cellulose wet gel ratios above 1:5, as the lignin precipitates during neutralization. At a ratio of 1:5, a water absorption of at least 186% was achieved, compared to 365% when no coating is applied. From a ratio of 1:50 onwards, a proportional decrease in water absorption can be observed with increasing lignin addition and increasing coating homogeneity. Fig. Figure 3 shows the water uptake of lignin-coated cellulose aerogel spheres with different lignin:cellulose wet gel ratios (L:C).

[0033] Using SEM technology, it is possible to visualize the adhesion of lignin to the surface and interior of cellulose aerogel spheres. Here in Fig. Figure 4 shows a clear difference between various L:C ratios. Lignin is clearly present as an amorphous layer that increasingly covers the surface of the cellulose aerogel with increasing L:C ratios. In the inner, porous network, which lies closer to the surface and is open to the outer cavities, small coated areas as well as thicker cellulose fibrils connected by lignin can be observed. However, these structures are unevenly distributed and are not observed in the core of the spheres. No lignin is observed there. Based on these results, it can be assumed that after dissolution in alkaline media and subsequent neutralization, lignin tends to interact with cellulose, but also with itself. This results in thin coating layers along the surface of the aerogel spheres without affecting their interior.This makes it more difficult for water to penetrate the interior of the spheres and reduces their water absorption. This will further affect the physisorption results. Fig. Figure 4 shows scanning electron micrographs of the interior (I) and exterior (E) of selected lignin-coated cellulose aerogel beads with different lignin:cellulose wet gel ratios (L:C).

[0034] As from Fig. As can be seen in Figure 5, all samples of the coated beads exhibit lower N2 adsorption values ​​than the uncoated sample. The maximum adsorption was observed at 1470 cm. 3 For the blank sample, this value drops to 748 cm at a ratio of 1:5. 3 / g. This decrease is proportional to the increase in added lignin. The more homogeneous filling of the surface mesopores leads to a general reduction in N2 access to the inner pores.

[0035] Fig. Figure 5 shows the N2 adsorption-desorption isotherm of lignin-coated cellulose aerogel beads with different lignin:cellulose wet gel ratios (L:C).

[0036] In the lignin-coated samples, a shift in the pore size distribution can be observed with increasing lignin concentration. This is in Fig. Figure 6 illustrates this. A decrease in pore volume can be observed for pores of all dimensions, particularly in the mesoporous range. At higher ratios (1:10 and 1:5), the profile begins to widen further, with a lower proportion of mesopores below 30 nm and especially between 30 nm and 50 nm. The average pore size remains stable regardless of the lignin:cellulose ratio (25.3 nm for the blank sample and 24.6 nm to 25.9 nm for the coated aerogel beads), indicating the homogeneity of the process. At this rate, lignin deposition on the bead surface is more intensive, and it occupies the pores that better match its molecular weight. Since fewer pores are available, water interacts less with these samples.

[0037] Fig. Figure 6 shows the pore size distribution of lignin-coated cellulose aerogel beads with different lignin:cellulose wet gel ratios (L:C).

[0038] In the thermogravimetric profile of the cellulose aerogels, a slight reduction in mass is attributable to the evaporation of water at 100°C. This would allow for the determination of the amount of water absorbed by the coated cellulose aerogel spheres in their moist state after 24 hours in a water bath. In the uncoated sample, water constitutes up to 73% of the sample mass, and a plateau is reached at 165°C before the cellulose degrades. When the sphere is coated with lignin in a 1:5 (L:C) ratio, the water content in the moist sphere is only 20% of the sample mass, with stabilization beginning at 104°C. The addition of hydrophobic lignin to the cellulose surface and the reduction in available pores result in lower water absorption. These data show that this invention can provide greater resistance to water absorption without making the cellulose aerogel hydrophobic.

[0039] Fig. Figure 7 shows the thermogravimetric profiles of lignin-coated cellulose wet aerogel beads with different lignin:cellulose wet gel ratios (L:C).

[0040] The development of lignin-coated cellulose aerogel beads can increase the attractiveness of this material for various industries, particularly the thermal insulation and filtration sectors. According to Lucintel's "Global Trends in the Thermal Insulation Market," the global thermal insulation market is valued at €46 billion and is projected to grow to €60.6 billion by 2025. Slightly more than half of this market is accounted for by the construction industry, followed by other sectors (oil and gas, automotive, etc.) and industrial applications. Foam insulation and glass wool are the most widely used thermal insulation materials and are expected to maintain their market share in the coming years. In Europe, glass wool and rock wool accounted for 58% of the market share in 2015, according to the European Commission (Competitive Landscape of the EU Insulation Industry for Energy-Efficient Buildings).According to Coherent Market Insights' "Market Analysis for Thermal Insulation Materials," the total value of the insulation materials market is projected to reach €92.8 billion by 2030. Aerogels' share of this market was estimated at €912 million in 2022 and is expected to grow to €2.76 billion by 2030, according to Spherical Insights' "Global Aerogel Insulation Market Size, Share, Forecast - 2032," particularly for silica aerogels. To increase the use of renewable resources in this sector, a rise in the share of bio-based components in the global thermal insulation market is anticipated. This could be based on the direct use of cellulose and other natural fibers, as well as the use of silica biopolymer composite aerogels. The utilization of raw materials and second-generation biomass waste is compatible with this approach and further enhances the sustainability of the production process.

[0041] Lignin-coated cellulose aerogel beads are highly porous, have a large surface area and low density, and a multifunctional surface that can interact with various organic components and metals. These properties and their biocompatibility make them interesting materials for filtration applications. The membrane filtration market was valued at €6.1 billion in 2022 and is projected to reach €9.3 billion in 2028, according to the MarketsandMarkets report "Membrane Filtration Market - Global Forecast to 2028".

[0042] According to the report "Global Aerogel Market 2018-2030" by Emergen Research, the aerogel market was valued at €657 million in 2020 and is projected to grow to €1.64 billion by 2030, with a primary focus on silica aerogels. Silica aerogels, along with organic and carbon aerogels, are increasingly used in various sectors, including construction, electric vehicle battery development, and the oil and gas industry. The growing share of bio-based aerogels is attributed to their applications in the biomedical field.

[0043] While previous techniques produced cellulose-lignin composite aerogels, this invention restricts the presence of lignin to the surface of the aerogels, while preserving the internal porous structure. This reduces the influence of moisture and water on the aerogel's structure and minimizes lignin leaching. The remaining lignin can be further functionalized to achieve higher hydrophobicity and fire resistance. This facilitates the industrial application and scale-up of bio-based aerogel production. While the aerogel industry has faced challenges in producing hydrophobic aerogels, this invention introduces a novel method for imparting moisture resistance to bio-based aerogels at minimal cost and with minimal reactant input.

Claims

[1] Method for coating cellulose wet aerogels with lignin by (a) Dissolving lignin in a basic aqueous solution, wherein the concentration of lignin in the aqueous solution is 0.1 wt% or more and the pH of the solution is in the range of pH >7 to 14, (b) Neutralizing the basic aqueous solution with organic acids or mineral acids, (c) Adding cellulose moisturizing gels, (d) then exchange of the solvent and (e) Drying with supercritical CO2. [2] Method according to claim 1, wherein the basic aqueous solution has a pH value of less than 13.5 and in particular of pH 9 to 13. [3] Method according to claim 1 or 2, wherein a sodium hydroxide solution is used in the first step. [4] Method according to any one of claims 1 to 3, wherein hydrochloric acid is used as a neutralizing agent. [5] Method according to any one of claims 1 to 4, wherein lignin with low or no sulfur content, in particular sodalignin, is used. [6] Method according to any one of claims 1 to 5, wherein the ratio of lignin : cellulose wet gel mass is not greater than 1:

5. [7] Method according to any one of claims 1 to 6, wherein regeneration, neutralization and solvent exchange take place at room temperature (20 °C) and normal pressure (1 bar). [8] Lignin-coated cellulose aerogels obtainable by a method according to any one of claims 1 to 7.

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