Bio-based Carbon Foam

Abstract

The present invention relates to bio-based carbon foams, their production methods and their uses. The method comprises foaming a slurry of cellulose fibers to obtain a cellulose fiber foam, adding a biomass component to the foam, and carbonizing the biomass-cellulose fiber foam.

Description

[Technical Field]

[0001] The present invention relates to bio-based carbon foams, methods for their production and their uses. [Background technology]

[0002] Macroporous and microporous materials are used in our daily lives in a variety of shapes and compositions. Carbon foams are porous materials formed from a network of carbon atoms and can have very large specific surface areas and high adsorption capacities. These materials are expected to contribute to modern technologies such as electrodes for electrochemical devices, absorbents for large molecules, insulation for aerospace components, and other applications that require lightweight, strong, and durable materials.

[0003] Known techniques for making carbon foam involve the mixing of various precursors. State-of-the-art precursors are typically based on fossil-based chemicals such as pitch, isocyanates, polyols, crosslinkers, chain extenders, and surfactants, while catalysts and other additives are inorganic. Each precursor requires different treatment during the foaming process.

[0004] The traditional method for preparing carbon foam is the template route, which involves using commercially available open-cell polyurethane foam as a template. The polyurethane foam is impregnated with a slurry of phenolic resin or pitch, then cured under inert gas and carbonized to form a carbon-foam structure. This method is simple and easily mass-produced, but both the template and precursor are fossil-based. Another method is direct foaming, which involves the generation of gas bubbles within a liquid slurry containing the precursor and blowing agent. The gas bubbles, usually CO2, are generated due to the chemical reaction of the blowing agent during the first step of the foaming process. Once a stable porous network is formed, it is completely dried under inert gas and carbonized. The direct foaming route uses large amounts of fossil-based precursors. A third method is indirect foaming, which involves preparing a precursor slurry and then passing nitrogen gas under pressure in an insulated reactor. The resulting foam is then dried and carbonized. This process is complex and expensive on a large scale.

[0005] There is growing interest in replacing fossil-based chemicals with more sustainable alternatives, including through the use of renewable raw material resources and in terms of environmental and human health. Cellulose is the most abundant renewable natural polymer on Earth and has particular potential because methods for its mass production on an industrial scale are available. Cellulose-based porous materials are typically produced by using an aqueous slurry of cellulose fibers as the starting material. The water must be removed during drying of the wet porous cellulose material without causing collapse or shrinkage. Recently, cellulose foams have been prepared from only biomass precursors, a bio-blowing agent, water, and air, as described in WO 2020 / 011587. WO 2020 / 049226 discloses porous moldable materials prepared from lignin-containing fractions obtained from lignocellulosic materials. US Patent Publication No. 3,894,878 discloses the preparation of porous moldable materials from aqueous solutions of lignin. However, there remains a need for environmentally friendly and inexpensive methods for preparing carbon foams. [Brief explanation of the drawings]

[0006] [Figure 1] Figure 1(c) shows a process for preparing lignin-cellulose-based carbon foam by (a) foaming a cellulose slurry to provide a cellulose fiber foam, (b) adding lignin (●) to the cellulose fiber foam (----), and (c) carbonizing the dried lignin-cellulose foam (in Figure 1(c) ○ represents carbonized lignin and -·-· represents carbonized cellulose fibers). DETAILED DESCRIPTION OF THE INVENTION

[0007] It is an object of the present disclosure to provide carbon foams made from bio-based materials.

[0008] In a first aspect, the present invention provides a method for producing a coating having a coating density of 10 to 80 kg / m 3 or 10 to 60 kg / m 3 and an average pore size of 0.5 to 10 mm or 1 to 10 mm.

[0009] Carbon foam according to the present invention has the advantages of being lightweight, non-flammable, chemically resistant, high temperature resistant, gas and liquid permeable, and sound and radiation absorbing.

[0010] In a second aspect, the present invention provides a method for producing a composition comprising: a) suspending cellulose fibers in a liquid medium to obtain a cellulose slurry; b) foaming the slurry to obtain a cellulose fiber foam; c) adding a biomass component to a cellulose fiber foam to obtain a biomass-cellulose fiber foam, the biomass component is selected from lignin and lignocellulosic materials; d) carbonizing the biomass-cellulose fiber foam to obtain a bio-based carbon foam; The present invention relates to a method for preparing a bio-based carbon foam, comprising:

[0011] The cellulose slurry in step (a) may be prepared by immersing dry cellulose fibers in a liquid medium followed by mixing using standard cellulose separation equipment. The liquid medium used to suspend the cellulose fibers may be an aqueous solution or water, preferably water. The suspension of the cellulose fibers and biomass components in the liquid medium may be carried out at a temperature of 10 to 50°C, 15 to 50°C, or 20 to 50°C. The concentration of the cellulose slurry may be 0.1 to 40 wt% cellulose, or 0.1 to 5 wt%, or 0.2 to 2 wt%, calculated based on the total weight of the cellulose and liquid medium in the slurry.

[0012] Additives may also be added to the cellulose slurry. The additives may be suspended in the liquid medium before the addition of the cellulose fibers, suspended together with the cellulose fibers, or added to the cellulose slurry after the cellulose fibers have been suspended. The additives may be foaming agents, thickeners, or plasticizers, and are preferably bio-based additives. Examples of bio-based foaming agents include proteins such as gluten, casein, hydrophobin, and gelatin. The additives may be added to the cellulose slurry as a powder or as a mixture to obtain a cellulose fiber and additive slurry having a total solids content of 0.1 to 40 wt.%, 0.1 to 5 wt.%, or 0.2 to 2 wt.%, calculated based on the total weight of the slurry. The ratio of cellulose fiber to additive may affect the degree of aggregation and separation of the cellulose fibers.

[0013] Foaming of the cellulose slurry may be carried out by introducing a gas into the slurry. The gas can be introduced into the slurry by bubbling or vigorous stirring, by adding a blowing agent, by pressurizing the slurry with a gas, or by direct introduction of the gas, such as bubbling the gas through the slurry. The gas introduced into the slurry may be selected from carbon dioxide, nitrogen, an inert gas, and air, or a mixture thereof. Preferably, the gas is air. Air can be introduced by bubbling or vigorous stirring, which is an inexpensive and direct method for producing foam. Foaming may be carried out at room temperature or at temperatures ranging from 5 to 100°C, 10 to 100°C, 10 to 80°C, 10 to 60°C, 10 to 40°C, 15 to 60°C, or 15 to 40°C. Foaming may be carried out at atmospheric pressure. Foaming may also be carried out by applying high pressure and then releasing the pressure, for example, by applying a pressure in the range of 102 to 500 kPa or 102 to 300 kPa. The volume of the slurry can increase by several hundred percent upon foaming, for example more than 200%, or even up to 1000% or more, depending on the amount of dry weight of cellulose. An advantage of the method according to the invention is that only moderate or no heating is required to obtain the cellulose fiber foam.

[0014] The biomass component may be added to the cellulose fiber foam in a dry particulate form such as a powder, or may be added to the cellulose fiber foam as particles dispersed in a liquid medium such as an aqueous solvent or water, forming a biomass dispersion. The biomass component is selected from lignin; and lignocellulosic materials such as wood flour, pulp, and wood fibers, e.g., dried lignocellulosic fibers, wood flour, fine-cut cellulose pulp, sawdust, and lignin powder; and various combinations thereof. The biomass component is preferably lignin. The lignin may be added to the cellulose fiber-containing foam in a dry particulate form such as a powder, or as particles suspended in a liquid medium such as an aqueous solvent or water. The average particle size may be within the range of 0.1 μm to 20 μm, 0.1 to 10 μm, 0.1 to 5 μm, or 0.1 to 1 μm. The weight ratio of biomass components to cellulose fibers is 3:1 to 1:3, preferably 2:1 to 1:2, more preferably 1:1, when calculated on dry material.

[0015] In some embodiments, the cellulose fiber foam is dried by removing the liquid medium before adding the biomass component. The cellulose fiber foam may be cast into a desired shape, such as a plate, a 3D structure, or a thin layer, before drying. The cellulose fiber foam may be dried until it contains less than 15% by weight, or less than 10% by weight, of the liquid medium, calculated based on the total weight of the cellulose fiber foam. In some embodiments, the first step in removing the liquid medium may be drainage, which can be facilitated by gravity or vacuum. Drying of the cellulose fiber foam can be carried out by various methods. Depending on the liquid medium used, the liquid medium may be evaporated by convection drying, by irradiation at room temperature, such as 15 to 30°C or 15 to 25°C, or by subjecting the cellulose fiber foam to an elevated temperature above 20°C, such as 25 to 130°C, 30 to 100°C, 30 to 80°C, or 50 to 80°C. Depending on the stability of the cellulose fiber foam, both low and high temperatures can be used. While moderate or no heating is required to obtain the cellulose fiber-containing foam, higher temperatures can shorten the time required to dry the foam. The residence time for drying the cellulose fiber foam may be 4 to 60 hours, 4 to 48 hours, 4 to 24 hours, or 4 to 12 hours, depending on the drying method used. Various ovens, such as baking ovens, curing ovens, drying ovens, vacuum drying ovens, industrial batch ovens, and continuous ovens, may be used. Heating the cellulose fiber foam from within using microwaves or a combination of microwaves and heat flow can further speed up the drying time. However, excessively high temperatures can cause the cellulose fiber foam to become non-uniform or damage the foam, resulting in peeling or structural collapse. The drying temperature can also be varied during drying; for example, a low initial temperature can be used with a gradual increase in temperature. The liquid medium can also be removed by solvent exchange; for example, water can be removed by solvent exchange using ethanol.

[0016] The dried cellulose fiber foam may be impregnated with a biomass component in the form of a biomass dispersion. Impregnation of the cellulose fiber foam with the biomass dispersion may take from 1 hour up to 16 hours. The impregnation may be enhanced by suction to ensure that the biomass dispersion reaches the deepest pores of the cellulose fiber foam. After impregnation and before carbonization, the resulting biomass-cellulose fiber foam may be dried to reduce the liquid medium content to less than 15 wt. % or less than 10 wt. % of the liquid medium, calculated based on the total weight of the biomass-cellulose fiber foam.

[0017] In another embodiment of the method according to the present invention, the biomass component is added as a powder to the cellulose fiber foam before drying the foam. The average particle size of the powder may be in the range of 0.1 μm to 20 μm, 0.1 to 10 μm, 0.1 to 5 μm, or 0.1 to 1 μm. Adding the powder to the cellulose fiber-containing foam at a low shear rate over a long period of time can prevent the foam's cells from collapsing, essentially maintaining the cell size. Adding the biomass component to the cellulose fiber-containing foam before drying allows the biomass-cellulose fiber foam to be cast into desired shapes, such as plates, 3D structures, or thin layers, before drying and carbonization.

[0018] In any embodiment disclosed herein, the biomass-cellulose fiber foam may be dried until it contains less than 15% by weight or less than 10% by weight of the liquid medium, calculated based on the total weight of the biomass-cellulose fiber foam. Drying of the biomass-cellulose fiber foam may be carried out by various methods, such as drainage, convection drying, or irradiation, or a combination thereof. Drying may be carried out at room temperature, such as 15-30°C or 15-25°C, or by subjecting the biomass-cellulose fiber foam to a temperature above 20°C, such as 25-130°C, 30-100°C, 30-80°C, or 50-80°C. While only moderate or no heating is required to obtain the biomass-cellulose fiber-containing foam, higher temperatures can shorten the time required to dry the foam. Drainage can be facilitated by gravity or vacuum. The residence time for drying the biomass-cellulose fiber foam may be 4 to 60 hours, 4 to 48 hours, 4 to 24 hours, or 4 to 12 hours, but may be adjusted depending on the drying method used. Various ovens, such as baking ovens, curing ovens, drying ovens, vacuum drying ovens, or industrial batch ovens and continuous ovens, may be used. The drying time can be further accelerated by heating the biomass-cellulose fiber foam from within using microwaves or a combination of microwaves and heat flow. However, excessively high temperatures can cause the biomass-cellulose fiber foam to become non-uniform or even damage the foam, causing structural delamination or collapse. The drying temperature can also be changed during drying; for example, a low initial temperature can be used and increased over time. The liquid medium can also be removed by solvent exchange; for example, water can be removed by solvent exchange using ethanol.

[0019] In any embodiment disclosed herein, the amount of biomass component in the biomass-cellulose fiber foam may be 20-70 wt %, preferably 30-60 wt %, and more preferably 40-60 wt %, calculated based on the total weight of the dry components in the foam.

[0020] In the method according to the present invention, the carbonization of the biomass-cellulose fiber foam is carried out by increasing the temperature in one or more steps to a maximum temperature T in the range of 700 to 1500 ° C at a rate of 1 to 100 ° C / min. max The carbonization can be carried out by increasing the temperature until the temperature reaches a desired value, more preferably 800-1300°C, and most preferably 950-1150°C. The total carbonization time, including cooling, may be 15-20 hours. Carbonization may be carried out under an inert gas, such as nitrogen, helium, neon, or argon, or a mixture thereof. An advantage of the method of the present invention is that carbonization can also be carried out on wet biomass-cellulose foam, such as foam from which the liquid medium has simply been drained. Carbonization of dried biomass-cellulose fiber foam essentially provides a carbon foam having the same pore structure as dried biomass-cellulose fiber foam. The wet biomass-cellulose fiber foam can be recovered and stored for later use or to be filled into a mold or formed into a desired shape by any other method, allowing for the preparation of a carbon foam having a predetermined shape after carbonization. Thus, one embodiment of the method of the present invention involves carbonizing a shaped biomass-cellulose fiber foam to provide a shaped carbon foam. After carbonization, the carbon foam may be left in the resulting shape or may be machined to the desired shape.

[0021] The method of the present invention allows the cellulose fiber foam to act as a porous template, providing mechanical and structural support for the biomass components during carbonization. The voids generated in the cellulose fiber foam can be maintained during mixing or impregnation with the biomass components and during carbonization, resulting in a low-density carbon foam. The resulting carbon foam after carbonization can, in principle, have the same porous structure as the biomass-cellulose fiber foam. An additional benefit of using a cellulose fiber-containing foam as a template is the small pore size and uniform pore size distribution of the final carbon foam. The amount and type of added biomass components allow for the adjustment of the pore size, pore characteristics, and final chemical composition of the carbon foam. The biomass components, and especially lignin, also prevent the foam from shrinking during carbonization. Overall, the method of the present disclosure allows for the adjustment of the physical properties and characteristics of the carbon foam, such as pore size, density, open area per volume, stiffness, and hardness.

[0022] The carbon foams prepared by the methods of the present invention may be modified with additional components, such as hydrophobic agents, activation gases, coating layers, or other chemicals, by thermal post-treatment, or a combination thereof. The carbon foams may be activated to provide the foam with desired properties, such as specific absorption properties or an increased specific surface area. Activation can be achieved by treating the foam with an activation chemical and heating at a temperature of 400-800°C, or by treating the foam with a gas at a temperature of about 800-1100°C. Suitable activation chemicals are selected from alkali salts, phosphoric acid, zinc chloride, and sulfuric acid, or mixtures thereof. The activation chemical can assist in removing residual moisture from the material. Suitable gases for activation are selected from water vapor and carbon dioxide, or mixtures thereof.

[0023] The present invention specifically encompasses a method for preparing bio-based carbon foam, comprising suspending cellulose fibers in a liquid to obtain a slurry, foaming the slurry to obtain a cellulose fiber-containing foam, adding lignin powder to the cellulose fiber-containing foam, optionally casting the foam into a plate, 3D structure, thin layer, etc., or other shape, drying the lignin-cellulose fiber foam, and carbonizing the foam.

[0024] The present invention also encompasses a method for preparing a bio-based carbon foam, comprising suspending cellulose fibers in a liquid to obtain a slurry, foaming the slurry to obtain a cellulose fiber-containing foam, drying the cellulose fiber-containing foam, dispersing lignin particles in a liquid medium such as an aqueous solvent or water to obtain a lignin dispersion, impregnating the dried cellulose fiber-containing foam with the lignin dispersion to obtain a lignin-cellulose fiber foam, drying the lignin-cellulose fiber foam, and carbonizing the lignin-cellulose fiber foam.

[0025] The advantages of the method of the present invention are that it is environmentally friendly, involves simple technology, and can be easily mass-produced. The main components used in the method are bio-based and renewable. The liquid medium can be an aqueous solution or water.

[0026] In a third aspect, the present invention relates to a bio-based carbon foam obtained using the method according to the second aspect of the invention. A further aspect of the invention is a monolithic bio-based carbon foam comprising a porous core having a density, the density of the foam increasing closer to its surface. The monolithic structure resembles that of mammalian bone and can therefore be used as a bone implant material and a scaffold for osteogenic cells.

[0027] Yet another aspect is the use of the bio-based carbon foam according to the present invention in electrodes for electrochemical devices, absorbents for large molecules, adsorbents, insulation for high temperature applications, aerospace components, energy storage, catalytic substrates, and stealth technology.

[0028] All words and abbreviations used in this application, unless otherwise indicated, should be construed as having the meanings commonly given them in the relevant art. However, for the sake of clarity, certain terms are specifically defined below.

[0029] In this disclosure, the term "bio-based material" refers to any material made from substances derived from living or once-living organisms or plants, including, for example, wood-derived materials, lignocellulosic materials, cellulose fibers, lignin, starch, proteins, polylactic acid, etc.

[0030] Cellulose is the main component in the cell walls of all plants. It can occur in different compositions depending on the type or part of the plant. In wood, for example, cellulose occurs together with lignin and hemicellulose. In leaves, cellulose occurs without lignin but with abundant hemicellulose. In cotton seed hairs, cellulose occurs in an almost pure form without lignin. Cellulose fibers suitable for preparing the carbon foam according to the present invention may be derived from wood, such as softwoods or hardwoods, leaves, or fiber crops (such as cotton, flax, and hemp). Suitable cellulose fibers may also be derived from regenerated cellulose, such as rayon and lyocell. Preferably, the cellulose fibers are derived from wood, and more preferably, they are pulp fibers obtained by a pulping process that liberates the fibers from the wood matrix. Pulp fibers can be liberated by mechanical pulping, such as thermomechanical pulp (TMP) or chemothermomechanical pulp (CTMP), or by chemical pulping, such as kraft pulp or pulp obtained by the sulfite process, soda process, or organosolv pulping process. More preferably, the cellulose fibers are pulp fibers liberated by a chemical pulping process. Even more preferably, the cellulose fibers are obtained from softwood kraft pulp or dissolving pulp. The cellulose fibers used in the present invention may be lignin- and hemicellulose-free. The various properties of each cellulose will affect the physical properties of the final carbon foam. Cellulose fibers are predominantly longer in length than in width. The average width of cellulose fibers may be 0.01 to 0.05 mm. The average fiber length of softwoods may be 2.5 to 4.5 mm, while the average fiber length of hardwoods may be 0.7 to 1.6 mm, and for eucalyptus, it may be 0.7 to 1.5 mm. However, fiber lengths can vary significantly at various growth locations, etc. The cellulose fibers used in preparing the carbon foams disclosed herein may have an average fiber length of 0.1 mm to 65 mm, 0.1 mm to 10 mm, or 0.5 mm to 65 mm, or 0.5 mm to 10 mm, or 0.5 mm to 7 mm.Varying fiber lengths can impart different mechanical properties to the material. Due to the length of the fibers, they can intertwine with one another, providing fiber-to-fiber bonds that provide strength to the foam structure. The aspect ratio, i.e., the ratio of fiber length to fiber width, of the cellulose fibers used to prepare the carbon foams of the present invention can be at least 10, at least 25, at least 50, at least 75, or at least 100, which preserves and stabilizes the foam structure during the drying procedure. The aspect ratio can be up to 6500, or preferably up to 2000.

[0031] The high aspect ratio, i.e., length to width ratio, of the cellulose fibers can impart flexibility to the final bio-based carbon foam. The predominant orientation of the cellulose fibers allows for the formation of different macrostructures in the foam that can affect outgassing during carbonization and the resulting carbonization kinetics, which can in turn affect the physical properties of the final bio-based carbon foam.

[0032] Lignin is a cross-linked phenolic polymer that provides rigidity by filling the spaces between cellulose, hemicellulose, and pectin components in cell walls, particularly in the formation of wood and bark. The polymer has no defined primary structure, and the exact chemical composition of lignin varies from species to species, but it is relatively hydrophobic and rich in aromatic subunits. Lignin is often an undesirable by-product in the production of cellulose, which is the world's largest renewable carbon source after cellulose. The specific average particle size of lignin for use in the present method is preferably in the range of 0.1 to 10 μm. Particles within the preferred size range provide good adhesion to cellulose fibers. For some applications, the ash content of the lignin should preferably be as low as possible.

[0033] The density of the carbon foam can be determined using equation (1): TIFF0007804656000001.tif12170 (where m is the mass of the piece of carbon foam and V is the volume of said piece of carbon foam.)

[0034] The void space in the carbon foams of the present invention may be in the form of interconnected pores, such that at least 50%, at least 70%, or at least 80% of the total void space volume of the porous material of the present invention may comprise interconnected pores. Average pore size and pore volume can be measured by conventional methods, such as by image analysis using microscopy. The porosity of the carbon foam may be in the range of 50-99%. The term "porosity", φ, refers to the total pore volume, V P and the total volume V of the carbon foam. Porosity can be measured by conventional methods, such as by image analysis using microscopy.

[0035] "Specific surface area" refers to the total surface area of ​​a solid material per unit mass and can be determined by pycnometry. The carbon foam of the present invention has a specific surface area of ​​500 to 1500 m 2 / g.

[0036] It should be noted that embodiments and / or features and / or advantages described in the context of one of the aspects and / or embodiments of the present invention may also be applied mutatis mutandis to all other aspects and / or embodiments of the present invention. [Example]

[0037] Features according to the present invention are further illustrated in the following examples.

[0038] material Dried cellulose foam was provided by Cellutech AB and softwood kraft lignin powder was obtained from Stora Enso AB.

[0039] Example Lignin powder is dispersed in water to obtain a lignin dispersion. Optionally, additives are added to increase the solubility of the lignin. Cellulose foam is impregnated with the lignin dispersion. The impregnated lignin-cellulose foam is dried and then carbonized. The density and pore size distribution of the foam are measured.

Claims

1. a) suspending cellulose fibers in a liquid medium to obtain a cellulose slurry; b) foaming the slurry to obtain a cellulose fiber foam; c) adding a biomass component to a cellulose fiber foam to obtain a biomass-cellulose fiber foam, obtaining a biomass-cellulose fiber foam, wherein the biomass component is selected from lignin and lignocellulosic materials; d) carbonizing the biomass-cellulose fiber foam to obtain a bio-based carbon foam; 1. A method for the preparation of a bio-based carbon foam, comprising:

2. 10. The method of claim 1, wherein the weight ratio of biomass components to cellulose fibers is from 3:1 to 1:3, calculated on dry material.

3. 3. The method of claim 1 or 2, wherein the liquid medium for suspending the cellulose fibers is water.

4. 4. The method according to claim 1, wherein the concentration of the cellulose slurry is 0.1 to 40% by weight of cellulose, calculated based on the total weight of the cellulose and the liquid medium in the slurry.

5. 5. The method of claim 1, wherein the cellulose fiber foam is cast into a desired shape.

6. The method of any one of claims 1 to 5, wherein the cellulose fiber foam is dried.

7. 7. The method of claim 6, wherein the dry cellulose fiber foam is impregnated with the biomass component dispersed in the liquid medium.

8. 5. The method of claim 1, wherein the biomass component is added to the cellulose fiber foam as a powder.

9. 10. The method of claim 8, wherein the biomass-cellulose fiber foam is cast into a desired shape.

10. The method of any one of claims 1 to 9, wherein the biomass-cellulose fiber foam is dried.

11. 11. The method of any one of claims 1 to 10, wherein the biomass component is lignin.

12. Carbonization is performed by increasing the temperature in one or more steps at a rate between 1 and 100°C / min to a maximum temperature T in the range of 700 to 1500°C. ｍａｘ 12. The method according to claim 1, wherein the temperature is increased until the temperature reaches 100° C.

13. 13. The method according to any one of claims 1 to 12, wherein the carbonization is carried out under an inert gas.

14. A method described in any one of claims 1 to 13, wherein the bio-based carbon foam comprises a porous core having a density, the density of the foam increasing closer to its surface.

15. 15. Use of the bio-based carbon foam obtained by the method of any one of claims 1 to 14 in applications selected from at least one of: electrodes for electrochemical devices, absorbents for large molecules, adsorbents, insulation for high temperature applications, aerospace components, energy storage, catalytic substrates, and stealth technology.

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