Porous spherical or shell-shaped aerogel particles or spheres of selected polysaccharides

Abstract

The invention relates to the production of porous spherical or shell-shaped aerogel particles or spheres of selected polysaccharides based on alginate, pectin, amidopectin, chitosan, carrageenan, and / or cellulose, wherein a solution containing a carbonate ion-containing solution of one or more of the polysaccharides is brought into contact with an acid-containing regeneration bath or an acid-containing solution of the polysaccharides is brought into contact with a carbonate ion-containing regeneration bath and then removed from the regeneration bath following the gelation process, the solvent is exchanged, and a drying process is carried out.

Description

[0001] Porous spherical or shell-shaped aerogel particles or beads of selected polysaccharides

[0002] The invention relates to the design of spherical or shell-shaped aerogel particles / beads made from selected low-density polysaccharides with hollow and / or foam structures. By encapsulating CO2 gas molecules in a wet gel network, hollow (core-shell) and / or foam structures can be created, which also include enclosed bubble structures.

[0003] In general, hollow structures are developed through sacrificial core templates, for example, solid (porogen or polymer particles), liquid (immiscible liquids controlled by emulsifiers), or gaseous (controlled carbon dioxide bubbles or various gas molecules), by layer-by-layer methods or self-assembly (by chemical modification or polyelectrolyte phase separation) (G.-D. Fu, GL Li, KG Neoh, and ET Kang, "Hollow polymeric nanostructures-Synthesis, morphology and function," Prog. Polym. Sci., vol. 36, pp. 127-167, 2011, doi: 10.1016 / j.progpolym- sci.2010.07. Oil; T. Mohan et al., "One-Step Fabrication of Hollow Spherical Cellulose Beads: Application in pH-Responsive Therapeutic Delivery," ACS Appl. Mater. Interfaces, vol. 14, pp. 3726-3739, 2022, doi : https: / / doi.org / 10.1021 / acsami.lcl9577; T. Sasaki, T. Kitagawa, S. Sato, S. Irie, and K.Sakurai, "Core / Shell and Hollow Polymeric Capsules Prepared from Calcium Carbonate Whisker," Polym. J., vol. 37, no. 6, pp. 434-438, 2005, doi : 10.1295 / polymj.37.434; Y. Liu, J. Yang, Z. Zhao, J. Li, R. Zhang, and F. Yao, "Formation and characterization of natural polysaccharide hollow nanocapsules via template layer-by-layer self-assembly," J. Colloid Interface Sei., vol. 379, no. 1, pp. 130-140, 2012, doi: https: / / doi.Org / 10.1016 / j.jcis.2012.04.058; H. Li, M. Wang, L. Song, and X. Ge, "Uniform chitosan hollow microspheres prepared with the sulfonated polystyrene particles templates," Colloid Polym. Sei., vol. 286, no. 6, pp. 819-825, 2008, doi : 10.1007 / s00396-008-1843-9; Y. Hu, Y. Chen, Q. Chen, L. Zhang, X. Jiang, and C. Yang, "Synthesis and stimuli-responsive properties of chitosan / poly(acrylic acid) hollow nanospheres," Polymer, vol. 46, no. 26, pp. 12703-12710, 2005, doi: https: / / doi.Org / 10.1016 / j.polymer.2005.10.110).

[0004] Recently, T. Mohan et al. (loc. cit.) described the preparation of hollow cellulose beads for drug delivery by deprotecting cellulose acetate beads at 90 °C with a 5M aqueous KOH solution. In this alkali hydrolysis process, complete deprotection occurred after 3 hours. The deprotected cellulose beads were then washed with water and air-dried. During this process, the particle size decreased from 2 mm to 950 pm.

[0005] Hollow polymethyl methacrylate and polystyrene particles were prepared by T. Sasaki et al. (loc. cit.) by applying a thin layer of the polymers to calcium carbonate whiskers by emulsion polymerization and later removing the core calcium carbonate by acid treatment.

[0006] Y. Liu et al. (loc. cit.) demonstrated the formation of hollow carrageenan-chitosan polyelectrolyte nanocapsules by layer-by-layer deposition of carrageenan and chitosan onto silicon dioxide nanoparticles and subsequent etching of the silicon dioxide core with hydrofluoric acid. Here, functionalized silicon dioxide nanoparticles were used as the core and sacrificial template for the formation of the hollow structure.

[0007] H. Li et al. (loc. cit.) used uniform sulfonated polystyrene particles as templates for the preparation of hollow chitosan microspheres. To maintain the intact shell structure, the chitosan was cross-linked with glutaraldehyde. The core, the sulfonated polystyrene particles, was removed by washing the particles with tetrahydrofuran and water. Y. Hu et al. (loc. cit.) prepared the hollow chitosan-polyacrylic acid nanospheres using a core-free template route, in which the mixture of acrylic acid and chitosan was cross-linked in a solvent medium to form the self-assembled hollow structures of the chitosan-polyacrylic acid nanospheres.

[0008] P. Eiselt et al., Biomaterials 21 (2000), pp. 1921-1927, describe porous supports based on alginate hydrogels for biomedical applications. BY Choi et al., International Journal of Pharmaceutics 239 (2002), pp. 81-91, describe the preparation of buoyant alginate beads for pharmaceutical applications. MAT Rasel et al., Dhaka Univ. J. Pharm. Sci. 11(1), pp. 29-35, 2012 (June), describe the formulation and evaluation of buoyant alginate beads containing sodium diclofenac. In all of the aforementioned cases, no aerogels were formed, even with the use of surfactants.

[0009] US Pat. No. 8,802,150 B2 describes a method for using sacrificial templates made of polymeric polystyrene beads, mesoporous silica, or diatomaceous earth with functionalized surface groups, to which alginate, collagen, chitosan, and other natural and synthetic polymers were applied. The shell structure was stabilized by crosslinking the polymeric functional groups, and removal of the core structure resulted in hollow spheres (Abhay Pandit, Gildas Rethore, Hemantkumar Naik, Yvonne Lang, and D. Finn, "Hollow biodegradable nanospheres and nanoshells for delivery of therapeutic and / or imaging molecules," US Patent Appl. 12 / 886492, 2010).

[0010] In contrast, the object of the present invention is to provide a simplified process for the production of porous spherical or shell-shaped aerogel particles or beads of selected polysaccharides.

[0011] The aforementioned object is achieved in a first embodiment by a process for producing porous spherical or shell-shaped aerogel particles or beads of selected polysaccharides based on alginates, alginic acids, pectinates, pectic acids, amidopectinates, chitosan, carrageenan, and / or cellulose, wherein a solution containing carbonate ions of one or more of the polysaccharide-containing solutions is brought into contact with an acidic regeneration bath, or an acidic solution of the polysaccharides is brought into contact with a regeneration bath containing carbonate ions, removed from the regeneration bath following gelation, the solvent is exchanged, and the mixture is dried. The process is preferably carried out without the presence of an emulsifier.

[0012] According to the invention, the CO2 gas is produced in situ from soluble additives of metal carbonates / bicarbonates by treating the carbonate-containing solutions of the underlying biopolymers of polysaccharides with acids. In this way, acidic salts of the corresponding cations, water, and CO2 gas molecules can be generated in situ. The size of the cavity, the composition of the shell, the thickness of the shell, and the uniformity of the gas bubbles can be controlled by adjusting parameters such as the concentration of the biopolymer, the concentration and pKa values ​​of the acid, the gel-inducing salts, and the concentration of the selected metal carbonates / bicarbonates.After the formation of the CO2 gas-encapsulated wet gel network, the hierarchical porous structures, physical properties, and optical properties can be varied by specifying the drying conditions, such as air drying of hydrogel and organogel (gel network containing organic solvents) and supercritical drying of the organogel body with alcohol, acetone, or CO2 gas-miscible organic liquids. These porous materials are suitable for applications such as separation, filtration, purification techniques, insulation, sound and microwave absorbents, transducers, dielectrics for electronics, mass transport, storage, supports, energy storage, encapsulation, catalysis, contrast agents for diagnostics, nanoreactors, sensors, lightweight components, bio-based hollow templates, and biomedicine.

[0013] Surprisingly, it has been found that spherical or shell-shaped hollow bodies are obtained in the process according to the invention. This was surprising, since the addition of calcium carbonate in the production of aerogels using the emulsion process is known to those skilled in the art (see Reference 1: Poncelet, D., Lencki, R., Beaulieu, C. et al. Production of alginate beads by emulsification / internal gelation. I. Methodology. Appl Microbiol Biotechnol 38, 39-45 (1992). https: / / doi.orQ / 10.1007 / BF00169416: or Reference 2: Poncelet, D., Poncelet De Smet, B., Beaulieu, C. et al. Production of alginate beads by emulsification / internal gelation. II. Physicochemistry. Appl Microbiol Biotechnol 43, 644-650 (1995). https: / / doi.org / 10.1007 / BF00164768: or Reference 3: PONCELET, D. (2001), Production of Alginate Beads by Emulsification / Internal Gelation. Annals of the New York Academy of Sciences, 944: 74-82. https: / / doi.Org / 10. llll / i.1749-6632.2001.tb03824.x)

[0014] A process for producing aerogel particles using calcium carbonate is described in CN116462883A. In the process described there, water and oil are emulsified. The acid contained in the solution is in the oil phase. When the calcium carbonate decomposes, calcium Ca 2+ is absorbed into the aerogel mold and stabilizes its structure. The resulting CO2 is in the oil phase and escapes as a gas. The structure of the aerogel mold is not affected. The process described there produces classic aerogel spheres—but these do not have a hollow structure.

[0015] Therefore, it was surprising to experts that during the production of aerogels, it is possible to capture the resulting gas in the molded body and thus create hollow bodies. Despite the porosity of aerogels, it is possible to enclose CO2 inside a structure.

[0016] The process according to the invention thus makes it possible to provide hollow bodies in the form of spheres or shells. These hollow bodies are aerogels based on polysaccharides, wherein the polysaccharides are selected from alginates, alginic acids, pectinates, pectic acids, amidopectinic acids, amidopectinates, chitosan, chitin, carrageenans, and / or cellulose. Particularly preferred for the purposes of the present invention are alginate, pectin, amidopectin, and / or cellulose as the polysaccharide.

[0017] A polysaccharide or mixtures of two or more polysaccharides are placed in a solution. The solution is, in particular, an alcoholic or aqueous or alcoholic / aqueous solution. Water and / or an alcohol, preferably ethanol, are preferred as solvents. Ethanol is particularly preferred as the solvent.

[0018] The carbonate ions preferably originate from alkali metal and / or alkaline earth metal salts. Alkali metal carbonates or alkali metal bicarbonates can be used as alkali metal salts. Calcium carbonate, magnesium carbonate, sodium bicarbonate, barium carbonate, and / or potassium bicarbonate are particularly used.

[0019] The carbonate salts decompose in the presence of an acid, i.e., when the pH changes. This produces CO2, which is then trapped by the polysaccharide(s) during the formation of the aerogel structure. The reaction rate of CO2 formation, on the one hand, and the rate of formation of a polysaccharide gel as a precursor to the aerogel, on the other, determine the exact structure of the resulting hollow body.

[0020] If the CO2 is produced quickly compared to gelation, a spherical aerogel particle is formed. Inside there are many small gas bubbles, as shown for example in Fig. 5. Fig. 5 (a) shows the SEM image of a spherical hollow aerogel sphere according to the invention from the outside (left) and in section (right), which was produced with a 0.1 M sodium bicarbonate solution. Fig. 5 (b) is an SEM image of a spherical hollow aerogel sphere according to the invention (left: external view, right: internal structure), which was produced with a 0.2 M sodium bicarbonate solution. The structure originates predominantly from the reaction between, for example, bicarbonate and acid. As a result of this rapid reaction, CO2 gas bubbles are formed which are surrounded by a thin outer aerogel shell. The faster the reaction, the thinner the outer shell (compare Fig. 5 (a) with Fig. 5 (b)).

[0021] If, however, the CO2 is released slowly compared to gelation, only a single gas bubble is trapped inside the hollow body. A shell-shaped aerogel particle is formed, as shown in Fig. 12. This structure is formed, for example, by the reaction of carbonate with acid. This reaction is slower than the reaction of, for example, bicarbonates with acids.

[0022] Due to the slow reaction between carbonate and acid, gelation of aerogel is comparatively fast. A thicker outer wall is formed, within which a gas bubble is located. This creates a more stable shell compared to spherical aerogel particles, which enclose multiple gas bubbles.

[0023] The rate and amount of CO2 formation can be adjusted by the concentration of carbonate and the pH value. The choice of carbonate salt or bicarbonate salt also influences the rate of CO2 formation. At the same concentration of a carbonate salt and a bicarbonate salt and otherwise identical reaction conditions, the bicarbonate (CO3) forms spherical aerogel particles with many small gas bubbles inside, while the carbonate (CCh) 2- ) leads to the formation of shell-shaped aerogel particles with a gas bubble inside.

[0024] In order for CO2 to escape from carbonate or bicarbonate, the pH must change. This can be achieved by adding a solution containing polysaccharide and carbonate ions to an acidic regeneration bath. Or one can have a solution containing both the polysaccharide and the acid and add this to a regeneration bath containing carbonate ions. The carbonate ions and acid are thus introduced separately and then come into contact with each other. Basically, the polysaccharide-containing solution is introduced and added to a regeneration bath. This addition is preferably carried out dropwise. For scaling, multiple nozzles can be used, optionally with a slight overpressure generated by compressed air or nitrogen gas. Other types of processes for producing polysaccharide droplets, which are well known to those skilled in the art, can also be used according to the invention.

[0025] By adding the polysaccharide-containing solution to the regeneration bath, gelation and thus the formation of aerogel particles from the polysaccharides is initiated and then proceeds progressively. After gelation is complete, the particles are removed from the regeneration bath, the solvent is replaced, and the particles are dried. These steps correspond to the usual manufacturing steps for aerogels, so the person skilled in the art is generally familiar with them.

[0026] Typically, the particles are first placed in water, allowing the solvent to be exchanged with water to remove ions and byproducts. If necessary, neutralization occurs with suitable basic compounds. Drying can occur under ambient conditions (to obtain xerogels), via alcogels (exchange of water with ethanol followed by drying at ambient conditions), using hexane (in alcogels, the ethanol is treated with nonpolar solvents such as hexane and dried at ambient conditions), or under supercritical CO2 conditions. The individual processes are explained in more detail later.

[0027] With a thick shell, swelling and shrinkage during purification and drying can be compensated for by the shell, ultimately maintaining the sphere's stability, even with alginates, especially at pH 1 or lower, so that the spheres form when the alginate drops are added. Lowering the temperature ensures a slower reaction and thus a better particle surface: larger specific surface area, smaller pore volume, smaller pore size.

[0028] Different acids can lead to different results; the reaction leading to the formation of the network is different, even when the same amount of CO2 is formed. The rate of CO2 formation influences the structure. Common organic or inorganic acids can be used as acids. If the polysaccharide is an acid, further acid addition is unnecessary. However, one can also be added to accelerate the formation of CO2. Suitable inorganic acids include hydrochloric acid, sulfuric acid, or other mineral acids.

[0029] With chitosan and carrageenan, acid is added to the solution before the product is placed in the regeneration bath. The carbonate salts were added to a regeneration bath. The pH of the regeneration was kept above 13, as the addition of polysaccharide solution lowered the pH. This embodiment is particularly suitable for the continuous production of hollow bodies. Here, the polysaccharide and acid are mixed and added to the regeneration solution.

[0030] Furthermore, with regard to chitosan, most inorganic and organic acids can be used to dissolve chitosan. The acids can contain acidic functional groups with varying degrees of protonation. Therefore, in this preferred embodiment of the invention, the counterions of the acidic functional groups in the solution can yield monoanionic, dianionic, trianionic, or multianionic salts of chitosan.

[0031] According to the invention, the solvent of the polysaccharide-containing solution and the solvent of the regeneration bath are miscible with one another. The process is carried out in a single solution. Both solvents are preferably selected from water and / or alcohol, in particular ethanol. Particularly preferably, neither the polysaccharide-containing solution nor the regeneration bath contain liquids that are immiscible with the solvents. The present process is not an emulsion process as known from the prior art, but rather a process for producing aerogel particles in solution. Therefore, in particular, no emulsifiers are required to enable thorough mixing. Preferably, therefore, no emulsifiers are present, although their presence does not impair the reaction and thus the formation of the hollow aerogel particles.

[0032] The regeneration bath is particularly preferably an aqueous solution.

[0033] According to the invention, the polysaccharide-containing solution and the regeneration bath can be at room temperature (20°C to 25°C). However, it is also possible according to the invention for one or both solutions to be heated to achieve better solubility of the polysaccharide. The temperature of the solutions, in particular of the polysaccharide-containing solution, is from 2°C to 80°C, preferably from 20°C to 60°C. The temperature of the regeneration bath is preferably from 2°C to 80°C, in particular from 20°C to 60°C.

[0034] Washing with water and / or alcohol, especially ethanol, can prevent the swelling and cross-linking of the particles.

[0035] In a further embodiment, the present invention relates to aerogel hollow bodies produced by the process according to the invention. The resulting hollow bodies have a BET surface area of ​​20 m 2 / g up to 600 m 2 / g and in particular a pore volume of 0.1 cm 3 / g and 5 cm 3 / g. Examples of implementation:

[0036] Materials and methods:

[0037] All chemicals were purchased from Sigma Aldrich (pectin, cellulose, sodium alginate, chitosan, kappa-carrageenan, all carbonate salts), Glentham Life Sciences Co Ltd. (chitosan), Across Organics Chemicals (chitosan), Machery-Nagel (cellulose), J. RETTENMAIER & SÖHNE GMBH + CO KG (amidopectin and pectin), Fischer Scientific (sodium hydroxide, potassium hydroxide, urea) and used as received. The products were characterized by shell density measurements (Micromeritics - GeoPyc 1360), skeletal density (Micromeritics - Accupyc II 1340; gas pycnometer - helium), BET analysis of the nitrogen adsorption-desorption isotherm (Micromeritics - Tristar II 3020), Fourier transform infrared spectroscopy (FTIR; Bruker Tensor 27 instrument), and scanning electron microscopy (SEM: Merlin - Carl Zeiss microscope; gold-sputtered samples). XRD measurements of the monolithic and powder samples were performed on a Bruker D8 DISCOVER diffractometer using Cu-Ko radiation (= 1.54 °A).Deionized water was used for all syntheses and washing procedures. Ethanol was used as a suitable solvent to replace water in hydrogels. Ethanol is a good solvent for supercritical drying processes. The carbonate salt concentration in moles according to the description refers to 1000 g of polysaccharide solution.

[0038] Example 1 :

[0039] Carbonate salts mixed with polysaccharide solution

[0040] In this example, hollow aerogel / xerogel particles were prepared from pectinate, pectic acid, amidopectinate, amidopectic acid, alginate, alginic acid, and cellulose, using the acidic medium as the regeneration bath and carbonate salts for CO2 gas generation in the matrix of the polysaccharide gel body. The polysaccharide solution was prepared in an aqueous medium and mixed with a metal carbonate salt, which can be soluble, partially soluble, or insoluble in an aqueous medium. The polysaccharide solution mixed with carbonate salt could be dropped into the regeneration bath or layered with two equivalent volumes of the regeneration bath to create various shapes of hollow core-shell structures, such as particles or monoliths, respectively. The regeneration bath could be a common organic acid and / or inorganic acid and could contain water, alcohol, and / or a mixture thereof.Additionally, inorganic salts were added to the regeneration bath to promote the formation of a strong gel network. In the case of alginate and pectin, divalent cationic salts were added as additives to form the intact gel network. The presence of divalent cations in the gel network after the formation of the hollow structure was confirmed by EDX spectra and FTIR spectroscopy (the carbonyl (-OC=O) vibrational bands were observed between 1400 and 1850 cm). -1Additionally, a copper(II) cation (e.g., copper diacetate) was used instead of calcium salt (since calcium salt always imparts a white color to the gel body) to observe color formation in the core-shell structure. The inclusion of copper ions for the formation of the gel network was confirmed in the formation of the alginate hollow structure. In the ice-cold state, the reaction between acid and carbonate salts was slower than at room temperature or higher. The formation of hollow structures is therefore highly temperature-dependent. The concentrations of polysaccharides and carbonate salts determined the thickness of the shell. Depending on the concentration of the reagents (carbonate salts, polysaccharides, acids, and additives) and the temperature, the size and shape of the cavities, the shell thickness, and the hollow microstructures could be varied. The drying conditions could be varied to alter the pore volume in the final core-shell structure.

[0041] Example 1.1 :

[0042] Pectin-based hollow aerogel particles

[0043] In this example, different pectin sources were used, differing in their degree of esterification (30 and 75% esterification; 75-85% galacturonic acid) and their viscosity due to their different molecular weights. In this case, the pectin concentration was varied between 0.5 and 10% w / w. Example 1.1.1:

[0044] Preparation of a solution of pectin with carbonate salts

[0045] An aqueous solution of pectin (2.5 wt%) is explained here. Pectin (2.5 g) was dissolved in water (77.5 g). After complete dissolution, the pH of the pectin solution (pH = 3–4.5, depending on the source) was neutralized to pH ~7. An aqueous alkaline solution (e.g., NaOH, KOH, or NH4OH; 0.05 M) was used to neutralize the pectin solution. A dispersion / clear aqueous solution of carbonate salt in 20 g of water was added to the clear neutralized pectin solution and stirred vigorously for 60 minutes. The weight of water in the final solution containing the pectin salts and the carbonate mixture was 97.5 g.

[0046] The addition of carbonate salts to the neutral pectin solution resulted in a clear, viscous solution or a semi-gel-like wet solid. When the pectin-carbonate-salt mixture became wet-solid (semi-gel-like), this mixture could be converted into a liquid form by heating. This meant that the mixture transformed into a thermotropic liquid form above 45 °C. Cooling the liquid reversibly converted the semi-gel into a wet solid. It could also be irreversible, depending on the concentration and the addition of carbonate salts.

[0047] Example 1.1.2:

[0048] Pectic acid hollow aerogel particles

[0049] The pectin solution containing carbonate salts was added dropwise to the regeneration bath with a pH <1, in which no divalent cations were present. The experiments were carried out with 0.5 to 1 M hydrochloric acid and sulfuric acid. In both cases, the particles were produced in a well-spherical shape. After washing twice with water, the wet-gel particles were washed four times with an ethanol-water mixture. The volume percentage of ethanol was maintained at 25. The amount of ethanol was then increased to 50, 75, and 100%. The wet-gel particles were then washed three times with ethanol and dried under different conditions (supercritical and ambient drying) to produce hollow particles.

[0050] Example 1.1.3:

[0051] Calcium pectinate hollow airgel particles

[0052] A clear solution of pectin and carbonate salt mixture was used to form the particle structures. An aqueous mixture of acetic acid (0.5 M) and calcium chloride (4 wt%) was used as the regeneration bath. The pectin-carbonate mixture was drawn up using a droplet-generating technique (e.g., conventional syringe-dropping technique), and the drops were allowed to fall into the regeneration bath. The drop height was adjusted to 1 to 7 cm to produce spherical particles and could vary depending on the viscosity and surface tension of the liquids. After the addition of the pectin solution, the regeneration bath was stirred for 20 minutes to complete the reaction and gelation. Since the CO2 gas bubbles were trapped in the gel body, the particles approached the upper surface of the regeneration bath. The particles were collected and washed with water.Solvent exchange and drying of the particles under different conditions (supercritical and ambient drying) resulted in hollow calcium pectinate aerogel particles.

[0053] Example 1.2:

[0054] Production of hollow bodies based on amidopectin

[0055] Amidopectin-based aerogels were prepared from amidopectin with an amidation degree of 18–23%, a methoxylation degree of 27–32%, and a polygalacturonic acid content of 65%. Amidopectin was used according to the same procedure as in the examples for pectin aerogels (Example 1.1). The amide (CO-NH2) bond was very stable, and no hydrolysis of the amide bonds was observed, which was confirmed by the FTIR data analyses. Example 1.3:

[0056] Alginate-based hollow aerogel particles

[0057] In this example, sodium alginate was used as the source, which was used in the preparation of alginic acid- and alginate-based aerogels. In this case, the alginate concentration was varied between 0.5–10 wt%.

[0058] Example 1.3.1 :

[0059] Preparation of a solution of sodium alginate with carbonate salts

[0060] The formation of hollow structures during the preparation of alginate beads is explained here for a 2 wt% alginate solution. The sodium alginate salt (2 g) was dissolved in deionized water (78 g). An aqueous solution of carbonate salt containing 20 g of water was added to the clear sodium alginate solution and stirred vigorously for 60 minutes. The weight of water in the final solution containing the alginate and carbonate salts was 98 g. The pH of the sodium alginate and carbonate mixture was varied between 7.5 and 11, depending on the concentration and alkalinity of the carbonate salts.

[0061] Example 1.3.2:

[0062] Hollow aerogel particles based on alginic acid

[0063] The alginate solution containing carbonate salts was prepared using a droplet technique (here using a conventional syringe technique). The alginate solution was then added dropwise to the regeneration bath with a pH <1 and without the presence of divalent cations. The experiments were carried out using 0.5 to 1 M hydrochloric acid and sulfuric acid. In both acid cases, the particles were produced with a good spherical shape. After washing twice with water, the wet-gel particles were washed four times with an ethanol-water mixture. The volume percentage of ethanol was maintained at 25%. The amount of ethanol was then increased to 50%, 75%, and 100%. The wet-gel particles were then washed three times with ethanol and dried under different conditions (supercritical and ambient drying) to produce hollow particles.

[0064] Example 1.3.3:

[0065] Alginate aerogel particles cross-linked with divalent cations

[0066] This example explains the production of calcium-crosslinked alginate aerogel particles.

[0067] The clear alginate solution was withdrawn using a droplet-generating technique (such as a conventional syringe-dropping technique), and the drops were allowed to fall into the regeneration bath containing acetic acid (~0.5 M) and calcium chloride (4 wt%). The droplet height was adjusted to 5 to 7 cm to produce spherical particles and could vary depending on the viscosity and surface tension of the liquids. After the alginate solution was added, the regeneration bath was stirred for 20 minutes to complete the reaction and gelation. Since CO2 gas bubbles were trapped in the gel body, the particles approached the upper surface of the regeneration bath. The particles were collected and washed with water. Solvent exchange and drying of the particles under different conditions (supercritical and ambient drying) yielded hollow alginate particles.

[0068] Example 1.4:

[0069] Cellulose-based hollow aerogel particles

[0070] Cellulose hollow gel particles were prepared using the following procedure. For the reference samples, a cellulose solution was prepared following the classical procedure. Commercial cellulose (products from Sigma Aldrich and Machery-Nagel) was used. The cellulose solution was used for the production of hollow structures or bubble-sealed monoliths. Example 1.4.1:

[0071] Preparation of a cellulose solution with carbonate salts

[0072] Cellulose powder (4 g) was mixed with aqueous sodium hydroxide (8 g NaOH in 60 g water) at room temperature. Stirring was carried out in a cold bath (-25 °C) for 60 minutes, inducing swelling of the cellulose. 4 g of urea was added to this highly viscous solution, and stirring was continued for 30 minutes. This caused the cellulose to swell, forming a highly viscous slurry. Meanwhile, an aqueous solution of urea (8 g urea in 20 g water) was prepared and mixed with carbonate salt (NC, 0.3 mol). This urea-carbonate salt mixture was added to the cellulose-NaOH-urea mixture and stirred for another 30 minutes. The entire mixture was then stored in the freezer for 18 hours. The cold mixture was brought to room temperature and stirred for 60 minutes. The concentration of cellulose and carbonate salts was varied depending on the final dimensions of the wet gel samples.

[0073] Example 1.4.2:

[0074] Cellulose-based hollow aerogel particles

[0075] A strong aqueous acidic medium was used as the regeneration bath. Sulfuric acid was the best choice for producing hollow particles, and the sulfuric acid concentration was varied between 1 and 2 M. The cellulose solution was withdrawn using a droplet technique (here, a conventional syringe-dropping technique), and the drops were allowed to fall into the regeneration bath. The droplet height was adjusted to 0.25 to 1 cm to produce spherical particles. It could be varied depending on the viscosity and surface tension of the liquids. Additionally, the regeneration bath was gently stirred for 20 minutes to complete the reaction. The CO2 gas bubbles containing the particles rose to the surface of the regeneration bath. The particles were collected and washed with water, the solvent was exchanged, and the particles were dried under different conditions (supercritical and ambient).

[0076] Example 2:

[0077] Carbonate salts mixed in the regeneration bath

[0078] Example 2.1.1 :

[0079] Chitosan-based hollow aerogel particles

[0080] The following describes the preparation of hollow chitosan particles. Carbonate or bicarbonate salts were prepared until saturated. To prevent the pH from falling below 13, the carbonate salts were added to the bath and the saturation level was maintained. In some cases, the alkali hydroxide solution was prepared together with the saturated carbonate salts.

[0081] A 3 wt% chitosan solution was prepared by treating chitosan powder (source: Glentam Life Sciences Co. Ltd., 95% deacetylation degree; Across Organics Chemicals, >75% deacetylation degree; Sigma Aldrich, >74% deacetylation degree) in distilled water with 3 vol% acetic acid. The clear solution was warmed to 30°C and then added dropwise to the potassium carbonate solution (2.5 M) at 40°C using a droplet technique (here, a conventional syringe-dropping technique). Gelation occurred immediately, and the ions diffused into the gel body, creating a cavity containing CO2 gas. The wet gel particles were collected and washed with 10 wt% ethanol in water until the gel was neutralized. The pH of the carbonate-based regeneration bath was maintained above 13. To maintain the pH over a longer period of time, 0.25 mol NaOH solution was prepared together with the carbonate salts or the carbonate salt was frequently injected into the regeneration bath.After neutralization, the hollow wet gel particles were washed with ethanol and supercritically dried or dried at room temperature. Example 2.2.1:.

[0082] Carbonate salts mixed in the regeneration bath

[0083] Carbonate or bicarbonate salts were prepared to saturation. To prevent the pH from falling below 13, the carbonate salts were added to the bath and the saturation level maintained. A potassium hydroxide solution (0.1 mol) was prepared together with the saturated carbonate salts. Potassium bicarbonate salts were used in the washing steps to lower the pH to neutral. Contact with pure water was avoided due to the high swelling capacity of kappa-carrageenan.

[0084] Example 2.2.2:

[0085] Kappa carrageenan airgel hollow particles

[0086] 3 g of acetic acid was added to the swollen / dissolved kappa-carrageenan (2.5 g) in water. The amount of water was 94.5 g. The mixture was stirred at 60 °C to obtain a clear solution. The temperature of the solution was then cooled to 45 °C. Meanwhile, the potassium carbonate solution (KC, 2 M) was prepared with KOH (0.1 mol), and the temperature of this regeneration bath was maintained at 45 °C. The warm acidic kappa-carrageenan solution was added dropwise using a droplet technique (e.g., with a conventional syringe). The hydrogel formed immediately, and a core-shell structure was formed through ion diffusion. The hydrogel particles were collected and washed with 0.2 M (three times) and then with 0.1 M (until neutralization) potassium bicarbonate solution. The particles were then washed with an ethanol-water mixture (20, 40, 60, 80, and 100 wt%). The particles were then washed three more times with ethanol.Supercritical drying of the particles yielded kappa-carrageenan aerogels. Results.

[0087] Pectin-, amidopectin-, alginate-, and cellulose-based polysaccharide solutions were prepared along with carbonate salts used as precursors. A variety of carbonate salts were used, each with different pKa values ​​and solubility properties in an aqueous medium. For example, potassium carbonate (KC) has a pKa value of 10.25 and is highly soluble in water, while potassium bicarbonate (KHC) has two pKa values ​​(10.3 and 6.35) and is five times less soluble than KC. Comparing sodium carbonate (NC) and KC, NC has the same pKa value as KC, but its solubility in water is about 3.5 times lower. Calcium carbonate (CaC) has a pKa value of 9 and is poorly or only slightly soluble in water in the presence of carbonic acid or at lower pH values. Therefore, in the present application, the differentiation of cations in the carbonate salts was taken into account when mixing the carbonate salts with polysaccharide solutions.Furthermore, the understanding of the rheological properties of the mixture of polysaccharide and carbonate salts revealed the possibilities for converting the precursors into gel-like, hollow / bubble-like structures.

[0088] Fig. 1 shows the comparative static shear viscosity data of the sodium alginate solution obtained from Example 1 by adding different concentrations of potassium bicarbonate, sodium bicarbonate, and calcium carbonate. As a reference data, a sodium alginate solution without carbonate / bicarbonate concentration was analyzed. The viscosity of the sodium alginate solution (2 wt%) was approximately 4.36 Pa-s. The addition of carbonate salts with various cations decreased the viscosity. Increasing the molar concentration of potassium bicarbonate (KHC) did not show a drastic decrease, even when the concentration increased fourfold. When calcium carbonate (CaC) particles were dispersed in the alginate solution, the viscosity of the solution became thinner and decreased compared to KHC.Sodium bicarbonate (NHC) appeared to have a lower interaction with the alginate polymer chain, resulting in a high viscosity (3.7 Pa-s) compared to the 0.05 mol concentration of KHC (3.3 Pa-s). In general, the rheological data showed that the addition of metal carbonates / bicarbonates reduced the viscosity values ​​of alginate.

[0089] The conventional syringe-dropping technique was used to produce particles or beads. The respective polysaccharide solutions were dripped into the gelling bath under gravity. When scaling up the process to produce particles, multiple nozzles were used with a slight pressure of compressed air or nitrogen gas on the polysaccharide solution.

[0090] According to Example 1, wet gel particles consisting of pectic acid, pectinate, amidopectinate, amidopectin, alginic acid, alginate, and cellulose were formed as soon as the liquid droplet touched the surface of the regeneration bath. The particles were intact and formed the shell, while ion diffusion occurred and the decomposition of carbonate molecules produced saturated CO2 gas bubbles in the core. As a result, the core-shell structure was formed.

[0091] Pectin-, amidopectin-, and alginate-based gels were prepared either in acidic form or in the form of the egg-case model bound to divalent cations. For these polysaccharides, gelation proceeded by two different pathways:

[0092] (a) in the absence of divalent cations and the pH of the medium <1 with formation of the hydrogen bonds of the -COOH and -OH groups (including the -NHz group in the case of amidopectin) or

[0093] (b) in the presence of divalent cations, which form ionic cross-linking and hydrogen bonds between the molecular chains (egg carton model).

[0094] Cellulose-based wet gels were prepared by treating the acidic regeneration bath with a NaOH-cellulose-urea-water mixture containing carbonate salts. This method resulted in simultaneous cross-linking of the particles and CO2 gas formation through decomposition of the carbonate salts. Therefore, hollow or bubble-sealed structures were created.

[0095] According to Example 2, the wet gel particles of chitosan and kappa-carrageenan were prepared by dropping the acidic polysaccharide solution into the carbonate salt regeneration bath. In the preparation of chitosan and kappa-carrageenan beads, the acidic polysaccharide solution was added to the alkaline potassium carbonate (KC) solution. In this case, the pKa value of KC was good enough to neutralize the acidic polysaccharide solution. Similar to Example 1, the intact core-shell structures were created by the decomposition of carbonate ions with the formation of CO2 gas in the core. The pH of the regeneration bath containing carbonate salts was >13. The addition of acidic polysaccharide solution lowered the pH of the regeneration bath below 13 due to the consumption and decomposition of the carbonate salts.To continuously produce the hollow wet gel particles and maintain the pH value >13, either 0.1-0.5 mol NaOH or KOH were mixed together with the carbonate salts in the regeneration bath or additional carbonate salts were added.

[0096] In the case of chitosan, when KC was preferably used in the regeneration bath because of its high water solubility compared to other carbonate salts, the temperature of the regeneration bath was increased to 30 to 40 °C.

[0097] In the case of kappa-carrageenan, the temperature of the carbonate salt-based regeneration bath and the acidic carrageenan solution was maintained at 40 °C to create stable hollow structures. The temperature range could vary between 20 and 80 °C depending on the concentration of kappa-carrageenan and the acid concentration. The presence of specific ions, such as potassium ions, in the regeneration bath was necessary to strengthen the wet gel body of kappa-carrageenan, as potassium ions promote the structural transition from a spiral to a double helix structure. For this purpose, 0.1 mol of KOH was mixed with carbonate salts in the regeneration bath. Drying:

[0098] The wet gels were exchanged with water to remove the ions and byproducts. After neutralization, four different drying methods were used:

[0099] (a) The hydrogels were dried under ambient conditions to produce xerogels. In this case, the samples became transparent, indicating the loss of pores during drying of the hydrogels at room temperature.

[0100] (b) To increase the porous structure, the hydrogels were treated with ethanol for solvent exchange. The ethanol concentration in the gel body was gradually increased by 20 wt% with each wash. Once the ethanol was 100% exchanged (alcogels), a portion of the sample was removed and dried under ambient conditions. Here, the samples were opaque, indicating that the pores in the shell structure had developed.

[0101] (c) The alcogels were treated with a nonpolar solvent such as hexane to increase the porous structure in the shell walls. The hexene gels were dried at room temperature.

[0102] (d) The alcogels were placed in an autoclave and dried under supercritical CO2 conditions.

[0103] Volume of the aerogel:

[0104] Compared to the reference (no carbonate salts used), the hollow polysaccharide materials increased in volume. This can also be described as expansion of the polysaccharide materials by inflating the core structure. Physisorption analyses:

[0105] Fig. 2 shows the physisorption properties of hollow / bubble-capped polysaccharide aerogels prepared by reacting various carbonate salts. In general, the specific (BET) surface area of ​​the polysaccharide aerogels ranged between 230 and 538 m². 2 / g and the pore volume between 1 and 5 cm 3 / g, depending on the structural backbone of the polysaccharide chain. Pectin-based aerogels, in particular, showed the highest values, i.e., a specific surface area of ​​up to 550 m 2 / g and a pore volume of up to 5 cm 3 / g. Although kappa-carrageenan-based aerogels showed a large volume shrinkage of up to 90-95%, the specific surface area was 275 m 2 / g and the pore volume at 1.8 cm 3 / g. The physisorption properties of the polysaccharide aerogels are summarized in Table 1.

[0106] Table 1: Physical properties of polysaccharide-based aerogels depending on different carbonates. Abbreviations in the text: HV = high viscosity; LV = low viscosity; DMeO = degree of methoxyesterification.

[0107]

[0108]

[0109] In most cases, the hollow structures of the polysaccharide aerogels exhibited low nitrogen adsorption and desorption properties and a high pore volume compared to empty polysaccharide aerogels. For example, the empty calcium pectin aerogels containing citrus pectin had a specific surface area of ​​476 m². 2 / g and a pore volume of 1.27 cm 3 / g, while the hollow structures produced using carbonate salt (KC) in the case of calcium pectinate have a specific surface area of ​​417 m 2 / g and a pore volume of 1.78 cm 3 / g and in the case of pectic acid aerogel a specific surface area of ​​351 m 2 / g and a pore volume of 1.71 cm 3 / g. Comparing the data with the hollow bodies produced using KC, the calcium pectinate salts produced with KHC showed a low specific surface area, but a high pore volume and pore diameter. Increasing the molecular weight of the pectin increased the specific surface area, as well as the pore volume and pore diameter. Furthermore, the physisorption data showed that the pore size of the BJH desorption was increased by adding the carbonate salts to the reaction medium compared to the empty aerogels (see Table 1). The calcium amido pectinate-based aerogels had a BET specific area of ​​423 m 2 / g, a pore volume of 1.85 cm 3 / g and an average pore diameter of 21 nm, which corresponds to the values ​​of calcium pectin aerogels prepared with a 0.2 mol concentration of KC.

[0110] The hollow bodies of the alginic acid aerogels had a low specific surface area of ​​223 m compared to the alginic acid aerogels produced in the absence of carbonate salts. 2 / g, a small pore volume and a small pore diameter.

[0111] Figs. 3 and 4 show the physisorption properties of alginate aerogels prepared with different carbonate salts. Alginate-based aerogels showed the same trend as pectinate-based aerogels. An exception was sodium bicarbonate (NHC), where the specific surface area decreased compared to the reference alginate aerogel (zero concentration of carbonate salt), while increasing the concentration of NHC salts did not cause a decrease in the specific surface area. Considering the rheological data, where NHC showed less interaction with alginate molecules compared to KHC, the influence of NHC on the self-assembly of alginate during network formation seemed to be very small. NHC produced a CO2-gas-generating core-shell structure, but the specific surface area was not affected. However, the BJH pore volume was increased from 1.23 to 2.12 μm. 1increased when comparing the addition of 0.05 and 0.2 mol NHC (see Table 1). It was clearly shown that blowing CO2 gas into the core structure compressed the macropores between the alginate fiber networks into mesopores and increased the BJH pore volume. Changing the divalent cation to copper showed a remarkably high specific surface area (527 m 2 / g), a high pore volume (4.6 cm 3 / g) and a large average pore diameter (35 nm) compared to the calcium alginate aerogel produced under the same conditions.

[0112] Cellulose, chitosan and kappa-carrageenan aerogels had a specific surface area of ​​more than 250 m 2 / g, which is typical for the nanofibrillated structure of polysaccharide aerogels.

[0113] Fig. 2 and 3 show the physisorption properties of the alginate aerogels prepared with potassium carbonate salts, which show a trend towards decreasing the specific surface area and increasing the BJH pore volume with increasing the molar concentration of potassium carbonate (KC).

[0114] Fig. 4 shows the physisorption properties of alginate aerogels prepared with different carbonate salts (potassium carbonate (KC), sodium carbonate (NC), potassium bicarbonate (KHC), sodium bicarbonate (NHC), calcium carbonate (CaC) and zero carbonate salts), showing the trend that the surface area values ​​decrease with increasing molar concentration of the different carbonate salts, with the exception of sodium bicarbonate.

[0115] Temperature and drying conditions influence the properties of hollow aerogel structures. Table 2, for example, shows the influence of temperature in the preparation of alginate hollow bodies. Reducing the temperature of the alginate solution and maintaining the temperature of the regeneration bath at room temperature resulted in a remarkably high specific surface area (385 m 2 / g), a small pore volume (0.71-0.86 cm 3 / g) and an average pore diameter (7.7 to 10.89 nm) compared to the experiments conducted at room temperature with both liquids. It was shown that the temperature of sodium alginate and KC salt can strongly influence the reaction with the regeneration bath containing calcium chloride and acetic acid and the gelation rate. Table 2: Physical properties of alginate aerogel as a function of temperature in the presence of KC (0.2 mol)

[0116] Table 3 below shows the effects of drying conditions on the properties of the alginate hollow bodies. Supercritical drying resulted in the production of aerogels with high specific surface areas, high pore volumes, and high average pore diameters. In most cases of ambient drying, the surface tension of the liquid within the pores influences the dried porous structured materials. The use of n-hexane, with its low surface tension and low vapor pressure, resulted in the formation of xerogels with high surface areas, but to a lesser extent than supercritically dried aerogels. Compared to water- and ethanol-filled hollow bodies, xerogels made from n-hexane exhibited high pore volumes and high average pore diameters, confirming that the pore structures between the nanofibrillated structures were preserved even after ambient drying.

[0117] Table 3: Physical properties of alginate-based aerogels made from beads under the influence of drying:

[0118] Microstructure analyses:

[0119] Fig. 5 shows exemplary scanning electron micrographs of a spherical alginate-based hollow aerogel sphere according to the invention, from the outside (left) and in section (right), which was produced with a 0.1 M sodium bicarbonate solution. Fig. 5 (b) is an SEM image of a spherical hollow aerogel sphere according to the invention (left: external view, right: internal structure), which was produced with a 0.2 M sodium bicarbonate solution.

[0120] Fig. 6 shows the calcium salt of amidopectin aerogels with a structure similar to pectin aerogels. Fig. 6 shows scanning electron micrographs of hollow structures of amidopectin aerogel particles containing calcium ions as cross-linking agents: (left) hollow spheres; (right) cross-section through the microstructure of a broken portion of a hollow amidopectin aerogel.

[0121] Fig. 7 shows the alginic acid-based (Figs. 7a-7d) and calcium alginate-based (Figs. 7e and 7f) aerogels. In the case of alginic acid, changing the regeneration bath from sulfuric acid to hydrochloric acid showed a remarkable change in microstructure. Hydrochloric acid as the regeneration bath developed a finely dispersed, tightly connected, nanofibrillar network (Fig. 7d), whereas sulfuric acid as the regeneration bath resulted in a finely dispersed, loose network with a larger volume of macropores. Calcium alginate aerogels exhibited a similar microstructure (Fig. 7f) to calcium pectinate aerogels, but no dimpled internal structure was developed (Fig. 7e). Fig.Figure 7 includes scanning electron micrographs of alginic acid-based (ad) and calcium alginate-based (ef) hollow structures prepared from an aqueous solution of sodium alginate with KC (0.2 mol) using different acidic regeneration baths: fractured hollow alginic acid aerogel prepared from a sulfuric acid regeneration bath without divalent cation (a) and its internal microstructure (b); fractured hollow alginic acid aerogel prepared from a hydrochloric acid regeneration bath without divalent cation (c) and its internal microstructure (d); fractured hollow calcium alginate aerogel prepared from acetic acid (0.5 M) regeneration bath with 4 wt% calcium chloride (e) and its internal microstructure (0-.

[0122] Figure 8 shows the microstructure of cellulose aerogels prepared in a sulfuric acid bath. The hollow cellulose particles were inherently brittle, and a large volume of macropores ranging between 50 nm and 1 pm was observed. Figure 8 includes scanning electron micrographs of hollow cellulose aerogel bodies.

[0123] Figure 9 shows the scanning electron microscope image of monolithic chitosan aerogels with fused hollow particles. The fractured surface revealed a hollow, spherical particle with a core cavity. The microstructure shows a connected nanofibril structure. Figure 9 includes scanning electron micrographs of chitosan aerogel beads prepared by washing the wet gels with water and fused into a monolithic structure. The jagged surface shows that the fused particles still have a hollow internal structure and an interconnected nanofibril network. Figure 10 shows scanning electron micrographs of hollow chitosan particles with a nanofibrillar microstructure. Figure 10 includes scanning electron micrographs of chitosan aerogel beads prepared by washing the wet gels with an ethanol-water mixture (1:9 vol.-%), which stabilizes the particles and prevents the formation of swellings that lead to particle fusion. The images show the bead structure and the fractured surface of the chitosan hollow particles. The bottom row shows the microstructure of the fractured inner surface.

[0124] Figure 11 shows the scanning electron micrographs of hollow kappa-carrageenan particles. Since the particles exhibited a volume shrinkage of 95%, they were nearly spherical and had a rough outer surface due to the shrinkage. However, the core-shell structure was intact, and the microstructure showed a coherent nanofibril network.

[0125] Figure 12 shows scanning electron micrographs of hollow bodies of calcium alginate aerogels prepared without surfactants by adding potassium carbonate salts. Figure 12a shows the 0.1 mol KC sample of the calcium alginate aerogel, which appears to be nearly spherical. Figures 12b to 12d clearly show that increasing the carbonate salt addition increased the core volume, reduced the shell thickness, and increased the total particle volume. The surface structure (Figure 12e) and the rugged internal structure (Figure 12f) exhibit randomly interconnected nanofibers, a characteristic feature of polysaccharide aerogels. Fig. 12 shows scanning electron micrographs of hollow spheres of calcium alginate aerogels prepared by adding potassium carbonate salts at a concentration of: (a and b) 0.1 mol, (c) 0.2 mol and (d) 0.5 mol.The surface microstructure shows nanopores (e) and the broken inner surface shows the interconnected nanofibril structures (f).

[0126] Fig. 13 shows scanning electron micrographs of hollow structures of calcium alginate-based beads prepared by air-drying wet gels with different liquids obtained from an aqueous solution of sodium alginate containing 0.2 mol KC: (a) water, (b) ethanol, and (c) hexane.

[0127] When the liquid in the pores is changed from supercritical CO2, n-hexane, or ethanol to water, the volume shrinkage appears to be either small or significantly increased in the same order. A comparison with the nitrogen adsorption and desorption analyses shows that the high volume shrinkage due to the evaporation of the liquid between the pore channels reduces the physical properties of the aerogels. However, the void in the core structure resulted in a lightweight structure with a porosity of 75-85%.

[0128] Fig. 14 shows FTIR data illustrating the quality of polysaccharide aerogels in the range between 4000 and 400 cm : (a) alginate aerogel, (b) alginic acid aerogel, (c) amidopectinate aerogel, (d) pectinate aerogel, (e) pectic acid aerogel, (f) methoxy ester-containing pectinate aerogel, (g) cellulose aerogel, (h) chitosan aerogel and (i) kappa-carrageenan aerogel.

[0129] The FTIR data indicate the quality of the produced hollow aerogel structures (see Fig. 14). The spectral data confirm the purity of the hollow structures produced in the process and show no impurities. In the case of the alginate-based aerogels, the alginate aerogels cross-linked with divalent cations showed strong vibrational bands at 1597, 1418, and 1298 cm 4 for the calcium-bound carboxylate functional group. For the alginic acid aerogels lacking the cations, the carboxylate functional groups were found to be characteristic of the symmetric and asymmetric stretching vibration bands at 1728 and 1597 cm, respectively. 4 The formation of pectin-based aerogels was determined by analyzing the vibrational bands at 1727 and 1592 cm 4 The formation of pectinate aerogels cross-linked with divalent cations was confirmed by the characteristic vibrational bands at 1600 and 1418 cm 4confirmed. During the production of pectin-based aerogels, the methoxyesterified carboxyl groups were partially hydrolyzed in the presence of weak carbonate salts such as bicarbonates, and the ester groups were chemically converted into functional carboxylic acid or carboxylate groups. In the presence of strong carbonate bases in pectin solutions, the methoxyesterified groups were completely hydrolyzed. These chemical changes can be observed in the FTIR fingerprint range at 1733 cm -1 The example of complete hydrolysis of methyl esters was found in the case of the use of potassium carbonate in the preparation of hollow structures, which is indicated by the absence of the vibrational band at 1733 cm -1confirmed in Fig. 14d and e. In the case of using sodium bicarbonate salts in the preparation of pectin hollow structures, the vibrational bands can be found at 1738 cm (see Fig. 14f) compared to Fig. 14d. In the case of amidopectin aerogels, the amido functional groups were still intact and no chemical change occurred (see Fig. 14c), but the ester groups were (partially or completely) hydrolyzed, similar to the pectin aerogels. In the case of cellulose aerogels, the hollow structures showed larger changes at 1420 cm, belonging to the -CH- bending vibrational band, which has been reported to be the characteristic sign of a change in the orientation of cellulose molecules at the molecular level.No change in the vibrational band was observed during the preparation of chitosan aerogels, indicating that no structural and chemical changes occurred during the preparation of the hollow structures. No chemical changes were observed for the kappa-carrageenan aerogels, and the FTIR spectral data were consistent with those published in the literature.

[0130] Furthermore, the quality of the hollow structures of the aerogels was confirmed by XRD powder analyses. In the case of alginate-, pectin-, and amidopectin-based aerogels and their derivatives, the hollow structures were poorly crystalline and exhibited very broad peaks. In the case of the cellulose aerogels, the crystallinity of the cellulose was completely converted from the cellulose I phase to the cellulose II phase, which could be due to the process of cellulose dissolution and the realignment of the molecules, forming a rigid structure through antiparallel arrangement. In the case of the chitosan aerogels, the crystallinity was not altered. In the case of the carrageenan aerogels, the hollow structures exhibited a completely amorphous structure, exhibiting a broad peak between the diffraction angles of 10 and 30° 2e. No impurities were found in any of the hollow aerogel structures according to FTIR and XRD data.

Claims

Patent claims 1. A process for producing porous spherical or shell-shaped aerogel particles or beads of selected polysaccharides based on alginates, alginic acids, pectinates, pectic acids, amidopectinates, chitosan, chitin, carrageenan and / or cellulose, wherein a solution containing carbonate ions of one or more of the polysaccharide-containing solutions is brought into contact with an acidic regeneration bath or an acidic solution of the polysaccharides is brought into contact with a regeneration bath containing carbonate ions, removed from the regeneration bath following gelation, the solvent is exchanged and dried.

2. Process according to claim 1, characterized in that the solution of the polysaccharide is added dropwise into the regeneration bath.

3. Process according to claim 1 or 2, characterized in that the carbonate ions originate from alkali metal and / or alkaline earth metal salts.

4. Process according to at least one of claims 1 to 3, characterized in that the polysaccharide-containing solution is free from emulsifiers.

5. Process according to at least one of claims 1 to 4, characterized in that the regeneration bath is free of emulsifiers.

6. Process according to at least one of claims 1 to 5, characterized in that the polysaccharide-containing solution and the regeneration bath are each free from liquids which are not miscible with the respective solvent.

7. Aerogel hollow body produced by the process according to at least one of claims 1 to 6.

8. Aerogel hollow body according to claim 7, characterized in that it has a specific surface area of ​​20 m 2 / g up to 600 m 2 / g.

9. Aerogel hollow body according to claim 7 or 8, characterized in that it has a pore volume of 0.1 cm 3 / g and 5 cm 3 / g.

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