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 and biomass components to obtain a biomass-cellulose fiber 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 technology, 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] FIG. 1(c) illustrates a process for preparing a lignin-cellulose-based carbon foam by (a) preparing a slurry of lignin (●) and cellulose fibers (----), (b) foaming the slurry to provide a lignin-cellulose fiber foam, and (c) carbonizing the lignin-cellulose fiber foam to obtain a carbon foam. (In FIG. 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 granular polymer having a 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 and biomass components in a liquid medium to obtain a slurry, the biomass component is selected from lignin and lignocellulosic materials; b) foaming the slurry to obtain a biomass-cellulose fiber foam; c) 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 slurry in step (a) may be prepared by soaking dry cellulose fibers in a liquid medium and then mixing them with the biomass components using standard cellulose separation equipment. Optionally, the biomass components may also be soaked with the dry cellulose fibers before mixing. The biomass components may be suspended in the liquid medium before adding the cellulose fibers, suspended together with the cellulose fibers, or suspended in the liquid medium after suspending the cellulose fibers. Separating the cellulose fibers in the liquid medium before adding the biomass components and then mixing facilitates uniform dispersion of the biomass components in the slurry. The uniformly dispersed biomass components in the slurry can provide a uniform foam in the next step. The liquid medium used to suspend the cellulose fibers and biomass components 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 slurry may have a total solids concentration of 0.1 to 40% by weight, or 0.1 to 5% by weight, or 0.2 to 2% by weight, calculated based on the total weight of the slurry.

[0012] The biomass component used in the method according to the present invention is selected from lignin; and lignocellulosic materials such as wood flour, pulp, and wood fiber, e.g., dried lignocellulosic fiber, wood flour, fine-cut cellulose pulp, sawdust, and lignin powder; and different combinations thereof. Preferably, the biomass component is lignin. The lignin used in preparing the slurry in step (a) may be in the form of particles. The average particle size 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. The weight ratio of the biomass component to the cellulose fiber, calculated on dry wood, is 3:1 to 1:3, preferably 2:1 to 1:2, and more preferably 1:1.

[0013] Varying conditions used in drying the slurry can affect the morphology of the final bio-based carbon foam, such as the possibility of obtaining a uniform foam or a monolithic foam with a porous inner core that densifies toward its outer boundary or surface. The amount of biomass component affects the carbonization yield and carbon content in the carbon foam, and may also allow for tailoring of the pore size, pore characteristics, and final chemical composition of the carbon foam. The biomass components, and lignin in particular, also prevent shrinkage of the foam during carbonization.

[0014] In the method according to the present invention, additives can be added to the biomass-cellulose fiber slurry. The additives can be suspended in the liquid medium before the addition of the biomass components and cellulose fibers, suspended together with the biomass components and cellulose fibers, or added to the slurry after one or both of the biomass components and cellulose fibers have been suspended. The additives can 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 can be added to the biomass-cellulose fiber slurry as a powder or as a mixture to obtain a slurry of biomass components, cellulose fibers, and additives with a total solids concentration 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 fibers and biomass components to additives can affect the degree of aggregation and separation of the cellulose fibers.

[0015] In the method of the present invention, the cellulose fibers and biomass components are mixed into the slurry before foaming, thereby enabling the formation of a foam with a uniform distribution of the biomass components. Foaming of the slurry can be carried out by introducing a gas into the slurry. The gas can be introduced into the slurry by bubbling or strong agitation, by adding a blowing agent, by pressurizing the slurry with a gas, or by direct introduction of the gas. The gas introduced into the slurry may be selected from carbon dioxide, nitrogen, and an inert gas, or air, or a mixture thereof. Preferably, the gas is air. Air can be introduced by bubbling or strong agitation, which is an inexpensive and straightforward method for creating foam. Foaming can 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. Foaming may be carried out at temperatures between 5-100°C, 10-100°C, 10-80°C, 10-60°C, 10-40°C, 20-60°C, or 20-40°C. The volume of the slurry upon foaming may increase by several hundred percent, e.g., more than 200%, or even up to 1000% or more, depending on the amount of dry weight of cellulose. The resulting biomass-cellulose fiber foam may be cast into desired shapes, e.g., plates, 3D structures, thin layers, etc.

[0016] Prior to carbonization, the biomass-cellulose fiber foam may be dried to reduce the liquid medium content, for example, to less than 15% by weight, or less than 10% by weight, calculated based on the total weight of the biomass-cellulose fiber foam. Drying the biomass-cellulose fiber foam provides a different pore structure than that in the wet cellulose-biomass fiber foam, e.g., larger pore sizes. Drying the biomass-cellulose fiber foam may be carried out by various methods, such as convection drying, irradiation at room temperature, e.g., 15-30°C, or 15-25°C, or by subjecting the biomass-cellulose fiber foam to elevated temperatures above 20°C, e.g., 25-125°C, 30-100°C, 30-80°C, or 50-80°C. High temperatures can shorten the time required to dry the foam. An advantage of the method according to the present invention is that only moderate heating, or even no heating at all, is required to obtain a foam containing dried biomass-cellulose fibers. 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, 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. Depending on the stability of the biomass-cellulose fiber foam, both low and high temperatures can be used. However, excessively high temperatures may cause the biomass-cellulose fiber foam to become non-uniform or may damage the foam, causing peeling or structural collapse. The drying temperature can also be changed during drying, for example, by using a low initial temperature and gradually increasing the temperature. The liquid medium can also be removed by solvent exchange, for example, by using ethanol to remove water.

[0017] 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 maximum of 800-1300°C, more preferably 950-1150°C. In a continuous furnace, heating and cooling can be carried out more rapidly than in a static batch oven. The total time for carbonization, including cooling, can be 5-20 hours, 10-20 hours, or 15-20 hours. Carbonization can be carried out under an inert gas, such as nitrogen, helium, neon, or argon, or a mixture thereof. Carbonization of a dried biomass-cellulose fiber foam can, in principle, provide a carbon foam with the same pore structure as a dried biomass-cellulose fiber foam. In another embodiment, carbonization can be carried out on a wet biomass-cellulose fiber foam.

[0018] The wet biomass-cellulose fiber foam can be collected and stored for later use or 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.

[0019] 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.

[0020] The present invention specifically encompasses a method for preparing bio-based carbon foam, comprising suspending lignin and cellulose fibers in a liquid to obtain a slurry, foaming the slurry to obtain a lignin-cellulose fiber 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.

[0021] Advantages of the method of the present invention include environmental friendliness, simple technology, and ease of mass production. The primary components used in the method are biobased and renewable. The liquid medium may be an aqueous solution or water. A further advantage is that the voids generated in the biomass-cellulose fiber foam can be maintained during carbonization, resulting in a low-density carbon foam. Mixing the cellulose fiber and biomass components prior to foaming allows for uniform distribution of the biomass components in the foam. A uniform biomass-cellulose fiber mixture also allows for uniform foaming throughout the mixture, resulting in a uniform final carbonized foam. An additional advantage of using a foam prepared from a slurry containing biomass components and cellulose fibers in a process for preparing carbon foam is the ability to create a flexible and resilient carbon foam.

[0022] In a third aspect, the present invention relates to a carbon foam obtained using the method according to the second aspect of the invention. A further aspect of the invention is a monolithic 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.

[0023] Yet another aspect is the use of the bio-based carbon foam according to the present invention in implants, electrodes for electrochemical devices, absorbents for large molecules, adsorbents, insulation for high temperature applications, aerospace components, energy storage, catalytic substrates, and stealth technology.

[0024] All words and abbreviations used in this application, unless otherwise indicated, should be construed as having the meaning commonly given to them in the relevant art. However, for the sake of clarity, certain terms are specifically defined below.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] The density of the carbon foam can be determined using equation (1): TIFF0007804655000001.tif13170 (where m is the mass of the piece of carbon foam and V is the volume of said piece of carbon foam.)

[0030] 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. The average pore size and pore volume may 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.

[0031] "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.

[0032] 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]

[0033] Example 1: Carbonized lignin / cellulose foams with different ratios of cellulose to biomass components Slurries are prepared by mixing various ratios of cellulose fiber and lignin in an aqueous solvent. Each slurry is foamed by mechanical whipping to obtain lignin-cellulose fiber foams. Each lignin-cellulose fiber foam is dried and then carbonized to obtain carbon foams. The resulting carbon foams are characterized by measuring various physical properties, such as average pore size and density.

Claims

1. a) suspending cellulose fibers and biomass components in a liquid medium to obtain a slurry, The biomass component is selected from lignin and lignocellulosic materials; suspending the mixture; b) foaming the slurry to obtain a biomass-cellulose fiber foam; c) carbonizing the biomass-cellulose fiber foam to obtain a bio-based carbon foam; wherein the weight ratio of the biomass component to the cellulose fiber is 3:1 to 1:3 calculated on a dry wood basis.

2. 10. The method of claim 1, wherein the liquid medium for suspending the cellulose fibers and biomass components is water.

3. 3. The method of claim 1, wherein the slurry has a total solids concentration of 0.1 to 40% by weight, calculated based on the total weight of the slurry.

4. 4. The method of claim 1, wherein the biomass component is in the form of particles.

5. The method of any one of claims 1 to 4, wherein the biomass-cellulose fiber foam is cast into a desired shape.

6. The method of any one of claims 1 to 5, wherein the biomass-cellulose fiber foam is dried before it is carbonized.

7. 7. The method of claim 1, wherein the biomass component is lignin.

8. Carbonization is performed by increasing the temperature in one or more steps at a rate of between 1 and 100°C / min to a maximum temperature T in the range of 700 to 1500°C. ｍａｘ 8. The method according to claim 1, wherein the temperature is increased until the temperature reaches 100° C.

9. 9. The method according to any one of claims 1 to 8, wherein the carbonization is carried out under an inert gas.

10. A method according to any one of claims 1 to 9, wherein the bio-based carbon foam comprises a porous core having a density, the foam being denser nearer its surface.

11. 11. Use of the bio-based carbon foam obtained by the method of any one of claims 1 to 10 in applications selected from at least one of implants, electrodes for electrochemical devices, absorbents for large molecules, adsorbents, insulation for high temperature applications, aerospace components, energy storage, catalytic substrates, and stealth technology.

Images