Modified biochar, methods for its production and applications thereof

Abstract

Subject of the invention are a modified biochar and methods for its production. In the method, biochar powder is modified in a chemical reaction with maleic anhydride, wherein the reaction is carried out at a temperature of at least 150°C, wherein the modified biochar is not subjected to carbonization after the reaction. Subject of the invention are also composite materials, shaped objects and construction parts, which comprise the modified biochar.

Description

[0001] Modified biochar, methods for its production and applications thereof

[0002] The invention relates to modified biochar and methods for its production. The inventions also relates to composite materials, shaped objects and construction parts which comprise the modified biochar.

[0003] State of the art

[0004] Various approaches are presently suggested and investigated for controlling the carbon dioxide content in the atmosphere. An especially effective approach is carbon (dioxide) sequestration and long-term storage in functional materials. It is desirable that such functional materials can be produced easily and conveniently, that they are of practical use, and that they can store high amounts of carbon. In this regard, construction and building materials are of special interest. Construction parts are used in high amounts, and the construction sector is responsible for a large share of total worldwide carbon dioxide release. Construction parts have the potential to store large quantities of carbon over long time periods.

[0005] Biochar is the lightweight black residue, made of carbon and ashes, which is obtained by pyrolysis of biomass. Pyrolysis is partial thermal degradation in an oxygen-controlled atmosphere. Biochar is widely used as a combustible and in agriculture to reduce runoff and increase soil fertility and crop yields. Since biochar consists mostly of carbon from the atmosphere, it is also a potentially interesting material for carbon sequestration and storage. Compared to other carbon storing organics such as carbohydrates, biochar displays a high recalcitrance, which prevents decomposition and re-entry of stored carbon into the atmosphere. As demonstrated by a growing body of data, biochar can remain stable under normal environmental conditions for hundreds of years.

[0006] Biomass from wood waste is one of the most abundant, cost-competitive, and sustainable biomass resources. An enormous amount of waste residue is generated in the process of making products such as lumber, furniture, pallets, and paper, where often less than 50 percent of the tree ends up in a final product. The remainder of this material is generally incinerated or landfilled, making it a highly cost-effective raw material, and an efficient source of biogenic carbon to produce biochar from. It has been proposed in the art to store carbon in composite materials from polymer materials and carbon-based fillers. If such composites could be provided for construction applications, carbon could be stored in large amounts for long time periods. Various composite materials have been described in the art in which biochar fillers are incorporated into polymer materials. A summary is provided in (11). Composites prepared by extrusion and moulding compositions comprising polyethylene and biochar filler are described in (12).

[0007] However, widespread commercial use of polymer composites with biochar filler is presently limited. The reason is that the preparation of such composites is difficult, and the composites tend to have undesirable mechanical properties and low stability. In this regard, it is a general problem that biochar powder is difficult to handle and process. For uniform compounding with polymers, biochar powder from particles in the lower micrometer range is required. Such a fine powder exhibits excessive dusting, which causes safety hazards and can pollute and damage devices for processing and compounding. The low density of dusty biochar particles makes handling even more difficult and prevents rapid processing.

[0008] Moreover, biochar has a poor compatibility with most organic polymers. However, the successful utilization of biochar-based fillers in durable polymer composites depends on uniform dispersion of the particles in the polymer and a strong interaction with the polymeric matrix. Therefore, it is difficult, or in many cases impossible, to distribute biochar filler uniformly in polymer compositions. Uniform liquid dispersions are especially required for moulding polymer composites. Because of the low compatibility, such polymer composites can have a non-uniform microstructure, become brittle and fracture. Overall, it is very difficult to prepare uniform and stable polymer composites comprising relevant amounts of biochar filler.

[0009] The problem is especially pronounced for biochar, which has been produced at high pyrolysis temperatures of 500°C or more. Such high pyrolysis temperatures are often used for producing wood biochar, which is relatively hard and dense. Biochar which is pyrolyzed at high temperature exhibits high chemical inertness. It has a higher carbon content and low surface functionality, and is very difficult to modify in chemical reactions. It is especially difficult to integrate such biochar into polymer composite materials. It would be desirable to modify the surface functionality of biochar, such that the uniform dispersion and affinity with polymer composites are improved. Further, it is desirable to reduce its water absorption and increase processability. However, for many varieties of biochar, especially those produced from woody lignocellulosic biomass at high pyrolysis temperatures, the chemical inertness provides a significant barrier for modification.

[0010] Many attempts have been made in the art to modify biochar to increase the hydrophilicity and compatibility with polymers. Various harsh treatments have been suggested in this regard. For example, strong acids or oxidizing agents such as hydrogen peroxide have been utilized to introduce reactive groups like hydroxyl (-OH) or carboxyl (-COOH) on the surface. However, these methods are of limited use, because they are unspecific, and can only introduce relatively low amounts of functional groups onto the biochar surface. Such harsh treatments are also not desirable for economic and environmental reasons.

[0011] CN 111117628 A discloses a method for preparing biochar material for soil repairing from straw. The method comprises an initial carbonization of straw at 300-400°C. The intermediate product is treated with a sodium hydroxide solution at 60-70°C. Maleic anhydride is added and the mixture is heated to 100°C to 150°C. Generally, maleic anhydride is rapidly hydrolyzed in sodium hydroxide solution. The intermediate solid product is separated and carbonized at 500°C to 600°C. In such a final pyrolysis step at relatively high temperature, the intermediate product is further carbonized, and functional groups would be decomposed. In general, such harsh treatments are suitable for providing activated biochar which has a highly porous structure and relatively large internal surface. However, functional groups cannot be attached selectively, or in significant amounts, to the biochar surface by such a harsh activation treatment.

[0012] CN 14605795 A discloses a biochar polylactic acid composite 3D printing material. The material is obtained from a reaction mixture comprising polylactic acid, biochar powder and a maleic anhydride grafted polylactic acid grafting agent. The document does not relate to reactions with maleic anhydride and surface-activated biochar.

[0013] It has also been suggested in the art to modify the biochar surface by more specific chemical reactions. However, because of the non-uniform structure and composition, and the low reactivity of biochar, it is very difficult to attach significant amounts of functional groups to the biochar surface by specific chemical reactions. Methods have been described in the art, in which highly uniform and ordered carbonaceous substrates, such as graphene, carbon black or activated carbon, could be modified with substituted alkenes. Graphene is an allotrope of carbon which consists of a single layer of atoms arranged in a hexagonal lattice nanostructure. Thus, it consists only of carbon and the structure is highly ordered. Carbon black also consists of carbon and has an essentially ordered structure of concentrically arranged, graphite-like crystallites. Activated carbon is another form of carbon that is extremely porous and has a large surface area for chemical reactions. The structure is characterized by flat surfaces of graphite-like material.

[0014] In a suitable reaction system, and under appropriate conditions, such ordered, uniform carbonaceous substrates can react in a [4+2] cycloaddition, also known as the Diels-Alder reaction. In this reaction, a conjugated diene group on the carbon substrate surface reacts with a substituted alkene, the dienophile. The alkene becomes covalently bonded the carbon surface upon formation of a cyclohexene ring.

[0015] However, it is known in the art that such cycloadditions have various drawbacks and limitations. Most importantly, Diels-Alder reactions are highly sensitive and require a specific electronic environment. They only occur in specific reaction systems, in which functional groups confer the required reactivity to the alkene (dienophile), such that it is adapted to the specific diene, and can undergo the reaction. Further problems are that cycloadditions tend to be sensitive to the reaction conditions, and are also reversible. It is another problem that the alkene can undergo alterative reactions, such as addition or acylation reactions. For these reasons, the use of Diels-Alder reaction for modifying substrate surfaces is limited.

[0016] It has been suggested in the art to react carbon black with maleic anhydride or maleic acid amide derivates (1). The reaction with maleic anhydride is carried out at 110°C in toluene solvent. The authors suggest that the reaction product is an [2+4] adduct, which is characterized by a cyclohexene ring structure, although no evidence is provided.

[0017] Graphene has been reacted with maleic acid derivatives or maleic anhydride (13). Graphene is characterized by a hexagonal lattice of carbon atoms. The authors note that the reaction is sensitive to the graphene sample and reaction temperature. The reaction is reversible, because the product is thermally instable. The reaction with maleic acid is carried out at 130°C in dichloromethane solvent.

[0018] Further, it has been suggested to react carbon black with maleic acid at 250°C for 3 hours, with or without a solvent (3). The product is acylated on the surface. Generally, acylated products can be hydrolyzed and have limited stability.

[0019] The structure of graphite nanoplatelets, which has been modified with maleic acid in a Diels- Alder reaction, is in (15). The reaction is carried out in the solid state in a kitchen blender at room temperature. The authors suggest that a cycloaddition occurs at the outer rim of planar graphite layers (Fig. 1).

[0020] The modification of activated carbon with maleic anhydride by Diels-Alder reaction is reported in (14). The reaction is carried out for 7 days at 120°C. The authors note that the Diels-Alder reaction on activated carbon surface is not completely understood, and that materials like fullerene, carbon nanotubes and graphene show different reactivity towards Diels-Alder reaction based on their aromatic structure.

[0021] In contrast, it has not been shown in the art that biochar can be modified in a cycloaddition reaction. Biochar is a crude product from natural origin. Biochar is less uniform and has a lower carbon content than other carbonaceous materials. Typically, the carbon content can be between about 50 wt.% to about 95 wt.%, wherein the remainder is O, H, S and N. The non-uniform structure, composition and surface chemistry could be the reason that attempts for modification by cycloaddition failed with biochar, which were reported in the art for ordered and uniform carbonaceous substrates. Without being bound to theory, it is assumed that biochar provides an electronic environment on its surface, which does not favor the electron transfer between the diene and dienophile, which would be required for a cycloaddition. Therefore, the prior art suggests, that biochar cannot be modified by a Diels- Alder reaction.

[0022] An overview about the functionalization of biochar with oxygenated groups is provided in (1). In the introduction, it is noted that carboxyl grafting via cycloaddition has been well- developed for enhancing the performance of carbon materials. However, there is still a “lack of comprehensive knowledge” how to tune oxygenated functional groups on biochar. Chapter 3.2 relates to the grafting of carbon substrates with carboxyl groups by acylation or cycloaddition. Accordingly, is has been reported in the art that the surface of biochar can be modified with maleic anhydride in an acylation reaction, as shown in Fig 4(a). Further, cycloaddition of fossil hydrochar with acrylic acid has been described. In Fig. 4(b), upper part, the reaction pathway for cycloaddition with maleic anhydride is postulated. However, it has never been reported in the art that such a reaction could be carried out. Therefore, the authors summarize in chapter 3.2 that a cycloaddition has only been reported for graphite, graphene, carbon nanotubes or activated carbon. The authors conclude that more efforts are needed to explore the potential of carboxyl grafting via the Diels-Alder reaction on promoting the performance of biochar. Overall, (1) confirms that it would be desirable to modify biochar by cycloaddition, but that such methods were not available in the art.

[0023] In conclusion, methods for modifying biochar by cycloaddition were not known in the art. Moreover, it was not expectable that such a reaction would be effective. In view of the specific structure and composition of biochar, and the sensitivity of cycloaddition reactions, the outcome of further modification experiments was at least not predictable.

[0024] There is a continuous need to provide improved modified biochar materials and methods for their production, which overcome the problems as outlined above.

[0025] Problem underlying the invention

[0026] The problem underlying the invention is to provide materials and methods, which overcome the problems of the prior art outlined above.

[0027] The problem underlying the present invention is to provide materials for permanently storing large amounts of carbon which has been sequestered from the atmosphere. The materials shall be suitable for construction and building applications. The materials should have high stability, such as mechanical, thermal and chemical stability. Preferably, the materials should have high density for efficient carbon storage, and low water uptake for high stability.

[0028] Further, the materials should be easily available from conventional raw materials at low costs. They should be available by convenient and simple procedures, which do not lead to safety problems during handling and processing. Overall, the materials should be suitable for mass production. It is a special problem underlying the invention to provide a method for modifying biochar, and especially highly inert biochar, which is from wood or has been pyrolyzed at high temperature. The method should efficiently activate the biochar, such that it can be used for preparing polymer composites. It is a specific problem to provide nanostructured modified biochar, which would be applicable for typical applications of nanomaterials.

[0029] Disclosure of the invention

[0030] Surprisingly, it was found that the problem underlying the invention is overcome by methods and products according to the claims. Further embodiments are outlined in the description.

[0031] Subject of the invention is a method for preparing modified biochar, wherein biochar powder is modified in a chemical reaction with maleic anhydride, wherein the reaction is carried out at a temperature of at least 150°C, wherein the modified biochar is not subjected to carbonization after the reaction. In the method, biochar powder and maleic anhydride are reacted at a temperature of at least 150°C. The biochar substrate is modified in a chemical reaction with maleic anhydride, in which maleic anhydride molecules are covalently bonded to the surface of the biochar particles. After the reaction, the modified biochar is not subjected to carbonization.

[0032] Maleic anhydride is the acid anhydride of maleic acid. It is advantageous that the compound is available in large amounts at relatively low costs. In a preferred embodiment, the maleic anhydride is from natural origin, and especially isolated from biological material. Thereby, the method and reaction product, and composites produced therefrom, can become more sustainable.

[0033] Biochar is a lightweight carbon-based material which is produced by pyrolysis of biomass, such as wood or other plant material. Pyrolysis is partial thermal degradation of the biomass in an oxygen-controlled atmosphere. The biochar can be grinded or milled into fine powder. Biochar is characterized by its high 13C isotope level, which distinguishes it from coal and char of fossil origin, reaction products and derivatives thereof. Typically, the carbon content is about 50% to 95 wt.%, based on all solid components

[0034] Initially, the inventors carried out a large series of experiments, in which it was examined if biochar can undergo a cycloaddition with maleic anhydride. Many different process conditions were tried, which had been described in the prior art for reacting ordered and uniform carbon substrates, such as graphene and activated carbon, with maleic acid anhydride and other unsaturated compounds. However, all these initial experiments failed. Under the conditions described in the art, it was not possible to graft maleic anhydride on biochar. The experiments confirmed the known fact that it is very difficult to modify biochar, because of its chemical inertness, irregular structure and composition, which are very different from ordered, uniform carbon substrates such as graphene and activated carbon.

[0035] However, to the surprise of the inventors, it was found in further experiments that an efficient reaction could be carried out at a temperature of 150°C or higher. This was unexpected, because corresponding reactions in the art were performed at lower temperatures, which also seems reasonable in view of the reversible nature of such reactions. However, the reaction products obtained at high temperature were also stable. It could be demonstrated that the modified biochar is the product of [4+2] cycloaddition. Moreover, it was found that the modified biochar can have a nanostructure. These findings are discussed further below and in the working examples.

[0036] In a preferred embodiment, the biochar and maleic anhydride react in a [4+2] cycloaddition (Diels-Alder reaction). In the Diels-Alder reaction, a ring is formed between a diene group on the surface of the biochar and a maleic acid anhydride molecule, as shown in the following scheme: diene dienophile Diels-Alder hydrolyzed Diels- adduct (1) Alder adduct (2)

[0037] The direct Diels-Alder adduct (1) is an anhydride. It comprises a five-membered ring with one oxygen atom from the anhydride group. The adduct (1) can be hydrolyzed to adduct (2), which is characterized by pairs of vicinal (adjacent) carboxyl groups on the biochar surface. According to the invention, the modified biochar is grafted with anhydride groups as product (1) and / or is the carboxylated product (2). In a preferred embodiment, the reaction product, preferably of the cycloaddition, is hydrolyzed. If the reaction is a Diels-Alder reaction, the modified biochar comprises vicinal carboxyl groups on the surface after hydrolysis. The carboxyl groups on the surface confer high compatibility with polymers to the modified biochar. The hydrolyzed modified biochar can also be stored and handled conveniently. The hydrolysis may occur at least partially during the reaction, or can be carried out after the reaction by adding water.

[0038] In an embodiment, the reaction product is the anhydride, i.e. the Diels-Alder adduct (1). The anhydride is a valuable precursor for forming the carboxyl product. It can also bind water at the surface, which can be advantageous in blends with hydrolyzable polymers.

[0039] In a further embodiment, the modified biochar is partially hydrolyzed. This can be advantageous because the carboxyl groups confer desired properties to the biochar, whereas the anhydride groups can bind water.

[0040] To the surprise of the inventors, the Diels-Alder reaction at high temperature is so efficient that it can be carried out with chemically inert biochar, such as wood biochar and / or biochar pyrolyzed at high temperature.

[0041] In a preferred embodiment, the biochar is wood biochar. Generally, biochar from wood is chemically relatively inert. Typically, it is relatively dense and hard, and has a specific surface structure. Therefore, it was unexpected that wood biochar can react with maleic anhydride in a cycloaddition reaction. It is advantageous that wood biochar can be efficiently modified in the inventive method. Woody biomass waste is one of the most abundant, cost- competitive, and sustainable biomass resources. An enormous amount of waste residue is generated in the process of making products such as lumber, furniture, pallets, and paper, where often less than 50 percent of the tree ends up in a final product. The remainder of this material is generally incinerated or landfilled, making it a highly cost-effective raw material, and an efficient source of biogenic carbon to produce biochar from. The inventive method provides an efficient and sustainable pathway for activating biochar, integrating it into composite materials and storing large amounts of carbon.

[0042] In a preferred embodiment, the wood is waste wood. Preferably, the waste wood is leftover pieces or saw dust from saw mills, or residue wood from forestry or garden, or recycled wood from furniture, building materials or the like. The use of waste wood is advantageous, because it renders the method and composite materials more sustainable.

[0043] In a preferred embodiment, the biochar substrate was prepared by pyrolysis at a temperature of more than 500°C, more preferably more than 550°C, or even more than 600°C, or more than 650°C. Depending on the pyrolysis temperature, the biochar particles can have different structural and chemical characteristics. When produced at lower pyrolysis temperature (<500°C), biochar particles possess a lower level of porosity and a higher amount of aliphatic functional groups. Biochar produced at high pyrolysis temperatures (>500°C) has a higher carbon content and reduced surface functionality. However, it was surprisingly found that such a biochar can be modified efficiently by the inventive method. Preferably, the carbon content of the biochar is relatively high. In this regard, it is preferred that the carbon content is at least 70 wt.%, more preferably at least 80 wt.%, or even at least 85 wt.%. Preferably, the carbon content is not more than 95 wt.%, or not more than 92 wt.%, or not more than 90 wt.%, which is approximately the upper limit obtainable by standard pyrolysis at high temperature. For example, the carbon content can be between 70 and 95 wt.%, or between 80 and 95 wt.%, based on all solid components. Most preferably, the carbon content is between 85 and 95 wt.% The amounts disclosed herein are in weight-% and based on total solids, unless stated otherwise. The carbon content is preferably determined by elemental analysis, for example according to EN13137:2001. Typically, such a high carbon content indicates that the biochar is chemically relatively inert and hard.

[0044] It is advantageous that the inventive method can be carried out with biochar which was pyrolyzed at such high temperatures. At first, it is generally advantageous that a simple and effective method is provided for modifying such an inert substrate. Moreover, modified biochar, which had been pyrolyzed at high temperature, is hard, durable and can bind high levels of carbon, which is advantageous for producing stable composite materials and environmental reasons.

[0045] In a highly preferred embodiment, the biochar is wood biochar, which was prepared by pyrolysis at a temperature of more than 500°C, or even higher as outlined above. The use of such biochar powder can provide especially stable and sustainable fillers and composite materials. In a preferred embodiment, the biochar powder has an average particle diameter of 10 pm to 2000 pm. Preferably, the particles have an average diameter of 50 pm to 1000 pm, more preferably between 100 pm and 600 pm. For example, a powder of desired particle size can be obtained by sieving. The diameter can be determined by Dynamic Image Analysis according to ISO 13322-2:2021-12. Biochar powder with particles of such sizes can be modified efficiently, and are suitable for compounding with organic polymers for preparing stable composite materials. Specifically, such biochar powders can be integrated in high amounts into the composite materials with organic polymers.

[0046] In a preferred embodiment, the reaction mixture comprises, based on the total amount of solids,

[0047] (a) 5% to 50 wt.% biochar, and

[0048] (b) 50% to 95 wt.% maleic anhydride.

[0049] Preferably, the reaction mixture consists of biochar, maleic anhydride, and optionally a solvent. Optionally, up to 20% additives can be included. The reaction mixture can be a dispersion of the solid biochar particles in molten maleic anhydride. Generally, it is preferred that the excess of maleic anhydride over biochar is relatively low. This is advantageous for an efficient reaction. Preferably, the ratio of biochar to maleic anhydride is 1 :10 or lower, more preferably 1 :6 or lower, based on the solid weights. Especially preferred is a ratio biochar to maleic anhydride between 1 :1 and 1 :10, preferably between 1 :2 and 1 :6, based on the solid weights. Typically, the reaction is effective at levels biochar to maleic anhydride between 5:1 and 1 :20, preferably between 2:1 and 1 :10.

[0050] In the inventive method, it is not necessary to include other components into the reaction mixture, such as functional additives or solvent. This is advantageous, because the method is simple, cost-efficient, and a highly pure reaction product is obtainable.

[0051] The reaction mixture may either comprise a solvent or may not comprise a solvent. In a preferred embodiment, the reaction mixture does not comprise a solvent. This is advantageous, because such a method is simple and cost-efficient. It is not necessary to remove residual solvent from the product. In another preferred embodiment, the reaction mixture comprises a solvent. In another preferred embodiment, the reaction mixture comprises a solvent. Preferably, the solvent has a boiling point above 180°C. It was found that an efficient reaction can be carried out when a solvent is included which has a relatively high boiling point. In this regard, the boiling point should be higher than the reaction temperature, such that the solvent does not evaporate. Preferably, the boiling point of the solvent is at least 20°C higher, preferably more than 30°C higher, than the reaction temperature.

[0052] Preferably, the solvent is an organic solvent. Preferably, the solvent is not miscible with water. Preferably, the solvent is selected from ethylene glycol, glycerin and aromatic solvents, such as toluene, or glycerin. Ethylene glycol (ethane-1 ,2-diol) is especially preferred, because it can mediate a highly efficient reaction. It is also advantageous that ethylene glycol is available from natural sources. Preferably, the solvent is not water and / or the reaction mixture is essentially water-free.

[0053] The inventive method requires a relatively high reaction temperature of at least 150°C. Preferably, the reaction temperature is at least 160°C, or even at least 170°C. It was found that the reaction is especially effective at about 180°C. Preferably, the reaction temperature is in the range of 150°C to 220°C, more preferably between 160°C and 200°C. If the reaction temperature would be higher, an undesired retro-Diels-Alder reaction may occur.

[0054] In a preferred embodiment, the reaction mixture is subjected to a temperature of at least 150°C for at least 2 hours. The reaction time is the time span at which the temperature of at least 150°C is maintained. It can be adjusted such that a desired degree of surface modification is achieved. Preferably, the reaction time is at least 2 hours, at least 5 hours or at least 10 hours. Generally, the degree of surface modification increases with reaction time. Whilst grafted carboxyl groups could already be determined after 6 hours, higher levels were determined after 12 or 24 hours. Preferably, the reaction time is between 2 and 36 hours, preferably between 4 and 24 hours.

[0055] Preferably, the reaction is carried out under mild conditions. In this regard, it is preferred that the pH value of the reaction mixture is between 4 and 10, more preferably between 5 and 9. Preferably, no acid or base is added to the reaction mixture, such as sodium or potassium hydroxide, HCI, sulfuric acid, nitric acid or the like, especially when the biochar substrate is reacted with the maleic anhydride. Such mild conditions are advantageous, because the anhydride is not hydrolyzed, and the maleic anhydride can completely react in the cycloaddition reaction. Thereby, undesirable side reactions, such as acylation of the biochar surface, can be prevented.

[0056] Preferably, the reaction is carried out at ambient pressure. It is advantageous that no specific devices are required for increasing the pressure. Preferably, the reaction is carried out under stirring. The reaction mixture is heated with a suitable device, for example in an oil bath.

[0057] Preferably, the reaction is a batch process and / or one-pot reaction. Preferably, it is carried out in a reaction vessel, which is connected to a condenser with a reflux system. In such an arrangement, the biochar powder is in intimate contact with the maleic anhydride, and a high level of modification is achievable. The amounts of biochar, maleic anhydride and optionally solvent, and reaction conditions, such as temperature, reaction time and the experimental set-up can be adjusted as known in the art. Overall, it is advantageous that the reaction system is simple and can react in a relatively simple device. In another preferred embodiment, the reaction is a continuous process. This is advantageous for large scale production of high amounts of modified biochar.

[0058] Typically, the modified biochar is obtained in powder form with a high level of anhydride and / or carboxyl groups on the surface. After the reaction, the modified biochar can be isolated and dried. Residual maleic anhydride and solvent can be removed by filtering. Preferably, the modified biochar is washed, for example with toluene or water, and dried. The thus obtained modified biochar is advantageous for compounding with polymers for producing polymer composites. In case the particles should form aggregates, they can be converted into a powder again, for example by milling or grinding.

[0059] It is advantageous that the inventive method is relatively simple. The modified biochar is obtainable in a single reaction step, if desired followed by hydrolysis step. It is not necessary to modify the biochar powder further, and no further reaction steps are required in the overall process. For example, it is not necessary to activate the biochar in a preceding or subsequent step, for example by partial oxidation with hydrogen peroxide or strong acid, or by further grafting other functional groups onto the biochar. In an embodiment, the modified biochar is not subjected to a subsequent reaction, in which the anhydride or carboxyl groups on the surface are reacted, especially esterified or amidated, to obtain a derivative of the modified biochar powder. However, the modified biochar may be subjected to subsequent reactions, if desired. For example, the carboxyl groups on the surface can be reacted, especially esterified or amidated, to obtain a derivative of the modified biochar.

[0060] After the reaction, and optionally hydrolysis, the modified biochar is not subjected to carbonization or pyrolysis (thermal decomposition). Preferably, the process does not comprise a step in which the modified biochar is heated to a temperature above 300°C, or above 400°C. Such post-treatments, which alter the surface structure, are not required, because the inventive method yields the advantageous product with a high degree of anhydride or carboxyl groups on the surface. In contrast, subsequent carbonization would decompose the functional groups which have been introduced on the surface in the inventive reaction.

[0061] Subject of the invention is also a modified biochar, which is obtainable by the inventive method as outlined above. The modified biochar is characterized by a unique structure and advantageous properties. The novel structure and the specific surface modifications are not obtainable by conventional methods.

[0062] Subject of the invention is a modified biochar, which comprises functional groups selected from

[0063] (1) and (2).

[0064] Preferably, the modified biochar comprises vicinal carboxyl groups on the surface. Vicinal carboxyl groups are bonded to adjacent carbon atoms of the biochar surface. The anhydride groups (1) are obtained directly in the [4+2] cycloaddition. The product with vicinal carboxyl groups (2) is obtained by hydrolysis of (1). The hydrolyzed product comprises vicinal carboxyl groups on the biochar surface. Such a structure is unusual and unique, because conventional carboxylation methods generally do not yield vicinal carboxyl groups on the surface, because the first carboxyl group can hinder subsequent introduction of a second vicinal carboxyl group. The vicinal carboxyl groups confer an unusually high polarity to the modified biochar. This can be advantageous for many uses, for example providing uniform blends with polymers or other matter.

[0065] Because of the surface structure, the modified biochar has a characteristic IR absorption, as determined by IR-spectroscopy. Typically, the modified biochar, which is hydrolyzed and comprises the vicinal carboxyl groups, shows a significant band with a peak between 1715 and 1725 cm1, specifically around 1719 cm-1, which is characteristic for the C=O stretch of carboxyl groups. Preferably, this band has the highest intensity in the IR range between 800 and 4000 cm-1. This indicates that the biochar carries a high amount of carboxyl groups on the surface, but lower amounts of other surface modifications.

[0066] Preferably, the hydrolyzed modified biochar shows a further band with a peak between 1155 and 1165 cm-1, specifically around 1159 cm-1. This band is characteristic for the C-0 stretch of carboxylic acid anhydride, and to a lower degree also of carboxyl groups. Preferably, the band for the C=O stretch has the highest intensity and that the band for the C-0 stretch has the second highest intensity in the IR spectrum between 800 and 4000 cm-1.

[0067] In a preferred embodiment, for the hydrolyzed modified biochar the intensity of the band for the C=O stretch is at least 2 times higher, preferably at least 5 times higher, than the intensity of the band for the C-0 stretch, and at least 10 times higher, preferably at least 20 or at least 30 times higher than the intensity of any further band between 2000 and 4000 cm-1, preferably between 800 and 4000 cm-1.

[0068] Preferably, the modified biochar shows no significant band in the range between about 3000 and 3600 cm-1. A band in this range would indicate the presence of alcohol groups (C-OH stretches). In this regard, “significant” can mean that the bands in this range are negligible, i.e. that the intensity of a band in this range is more than 10 times lower, preferably more than 20 or 30 times lower than the band for the C=O stretch. Typically, such bands for alcohol groups are present in biochar which was oxidized unspecifically, for example with hydrogen peroxide or strong acid. Preferably, the IR spectrum of the hydrolyzed modified biochar between 800 and 4000 cm-1is approximately as in Fig. 1. Such an IR absorption is only obtainable by the highly specific reaction according to the present invention.

[0069] Preferably, the modified biochar is not the product of an acylation. Therefore, it is preferred that the maleic anhydride is not bonded to the biochar surface via a single ester bond. The product of such an acylation reaction is shown in (1), Fig. 4(a). It has been described in the art only for hydrochar, which is of fossil origin. However, in this embodiment, the modified biochar may comprise a low amount of acylated functional groups, which can be formed in side reactions.

[0070] Typically, the modified biochar has a relatively high level of oxygen on the surface. Preferably, the O / C-ratio on the surface is at least 0.4, more preferably at least 0.5, even more preferably at least 0.6. Preferably, the amount of oxygen on the surface, based on the total of elements, is at least 30%, more preferably at least 35%, for example between 30% and 70%. The amount of oxygen and O / C-level on the surface can be determined by X-ray photoelectron spectroscopy (XPS), for example as described in Krishnamoorthy et all, Carbon 53 (2013): 38-49.

[0071] In the reaction, it was observed that the amount of oxygen on the surface is significantly increased. Preferably, the amount of oxygen on the surface of the biochar substrate is relatively low, such as between 5% and 20%, and increased to a level between 30% and 70% for the modified biochar.

[0072] In view of the unreactive nature of the biochar substrate, it was surprising that the inventive method is suitable for grafting carboxyl groups onto the surface of biochar particles. Moreover, to the surprise of the inventors, it was also found that the inventive method can modify the shape of the biochar particles. Specifically, it was found that the method can confer nanostructures to the biochar.

[0073] Preferably, the modified biochar is nanostructured. A nanostructure has an intermediate size between microstructures (in the micrometer range) and molecular structures. Preferably, the nanostructure comprises structural elements having dimensions between 0.1 and 100 nm. For example, the nanostructure can be a texture or surface pattern, and / or the modified biochar can comprise nanoparticles.

[0074] In a preferred embodiment, the modified biochar comprises nanoparticles. Nanoparticles can be formed, if the biochar particles are degraded at least in part during the inventive method. Preferably, the nanoparticles have a diameter and / or average diameter in the range of 1 to 900 nm, more preferably between 10 and 500 nm. Preferably, the biochar particles exfoliate in the inventive method at least partially. Without being bound to theory, it is assumed that the treatment with maleic anhydride can at least partially “break down” biochar particles. It is assumed that the structural disintegration of the larger biochar particles synergistically benefits the surface grafting process, vastly increasing the grafting density. In the reaction mixture at elevated temperature, it seems that the maleic anhydride can simultaneously exfoliate larger biochar particles to form smaller nanostructures, provide a stabilizing agent for the formed structures, and act as the dienophile in the Diels-Alder cycloaddition.

[0075] It has been suggested in the art that electrons in a graphitic layer can exhibit a honey-like viscous flow through its plane, caused by electron-electron collisions. Since graphite structures comprise unpaired TT-electrons uniformly resonated on the graphene plane, it is hypothesized that high-energy collisions could induce an electron flow towards the edge. This would result in a temporary local negative charge at the plane edge, when the graphitic sheets undergo breakage. In the inventive method, it is assumed that the maleic anhydride can cause the exfoliation and fracture of the graphic sheets within the larger biochar particles by an intercalation exfoliation mechanism. Here, maleic anhydride can react as a dienophile with these electron-rich active sites. Thereby, the maleic anhydride can stabilize these active sites through its LIIMO orbitals by forming sigma bonds with highly-activated HOMO orbitals of the graphitic sheet edges. The high porosity of the larger biochar particles may further enhance this process, ensuring the maleic anhydride has efficient access to carbonaceous sheets contained within the biochar.

[0076] Evidence of nanostructures can be provided indirectly by a change of optical properties, for example fluorescence of the modified biochar upon illumination with UV light, or if the color of the modified biochar is different from the black of the biochar substrate. In a preferred embodiment, the optical properties of the modified biochar are different than for the biochar substrate. Preferably, the colour of the modified biochar is different from black. More preferably, the colour is brown or brownish. Preferably, the colour is clearly brown, or coffeelike, and not only a shade of brown. Preferably, the colour is different from the initial black of the biochar substrate. In a preferred embodiment, the modified biochar is fluorescent, especially when dispersed in water and illuminated with UV light. In these embodiments, the optical properties are conferred to the biochar by the treatment with maleic anhydride, and not by other means such as additives. The formation of nanostructures is advantageous, because the modified biochar can be used for applications in which nanoscale structures and typical properties of such materials are desired. Further, the nanostructure is conferred to the biochar simply during the inventive method. Thus, it is not required to apply conventional treatments, such as milling or grinding, for obtaining nanoscale structures. The inventive method is also advantageous compared to such techniques, and provides different materials, because the resulting particles do not have typical cleavage structures from milling, and because the nanostructures become fully accessible to the reaction with maleic anhydride during the method.

[0077] The modified biochar is preferably a powder. This is advantageous for subsequent applications, especially for preparing polymer composites. The average particle diameter may be smaller than of the biochar powder substrate.

[0078] Subject of the invention is also a composite material comprising a polymer matrix, in which the modified biochar of the invention is distributed. Preferably, the biochar particles are dispersed uniformly throughout the composite. Preferably, the modified biochar functions as a filler. The carboxyl groups on the particle surface can render the modified biochar compatible with the organic polymer and support intimate blending. A composite material is obtainable, which can have a defined form, high mechanical stability, and a high ratio of biochar filler which is evenly dispersed in a polymer matrix.

[0079] Preferably, the organic polymer is thermoplastic or thermoset. Preferably, the organic polymer can be moulded. For example, the polymer can be a polyamide, polyolefin, such as polyethylene or polypropylene, polyester, polyurethane, silicone, acrylic polymer or rubber, or a copolymer or mixture thereof. Preferably, the polymer is a synthetic polymer, which can be advantageous for moulding. Preferably, the polymer is recycled and / or biobased, which is advantageous for the overall environmental balance.

[0080] In a preferred embodiment, the polymer is thermoplastic. When thermoplastic polymers are used, the composite is obtained from a polymer melt, in which the modified biochar is dispersed. For example, the thermoplastic polymer can be polyolefin, polyamide, acrylonitrile butadiene styrene (ABS), polylactide, or mixtures thereof. Preferably, the thermoplastic polymer is polyethylene, polypropylene or polyamide. The modified biochar is compatible with such thermoplastic polymers, such that shaped objects with high mechanical stability can be obtained therefrom by moulding.

[0081] In a preferred embodiment, the polymer is hydrophobic. Since the modified biochar is compatible with hydrophobic polymers, such as polyolefins and polyamides, stable composites can be produced from such polymers which comprise high levels of biochar.

[0082] In another preferred embodiment, the polymer is a polyolefin, such as polyethylene or polypropylene. Preferably, the thermoplastic polymer is polyethylene. The polyethylene can be HDPE (high density polyethylene). This is advantageous, because polyethylene is available in high amounts, at low costs, and can be recycled conveniently. Thus, respective materials are suitable for large scale building applications and permanent carbon storage.

[0083] In a preferred embodiment, the organic polymer is susceptible to hydrolysis. The modified biochar is especially suitable as a filler for such polymers, because it tends to bind residual moisture in the channels in the interior, thereby preventing hydrolytic degradation of the polymer. Preferably, the thermoplastic polymer is polyamide. The polyamide can be aliphatic polyamide, polyphthalamide or aromatic polyamide. Preferably, the polyamide is aliphatic, such as PA6 or PA66. Such polyamides are susceptible to hydrolysis and can be stabilized by the modified biochar filler, such that the composite material can remain stable over long time periods.

[0084] In another embodiment, the organic polymer is a thermoset. A thermoset, such as a rubber, comprises reactive groups for curing. The reactive groups, such as epoxy, vinyl, diene or silyl groups, or sulfur in any form, form covalent bonds upon curing. Thereby, the polymer is crosslinked and forms a covalent matrix.

[0085] In a preferred embodiment, the organic polymer is not curable and / or cured. Preferably, the polymer does not comprise reactive groups such as hydroxyl, carboxyl or amine groups, which are reacted when the biochar and shaped object are prepared. According to the invention, stable composite materials can be obtained even when the polymer is not cured. This is advantageous, because systems with curable polymers are generally more complicated and more difficult to control, whereas the products are more difficult to recycle. In a preferred embodiment, the organic polymer is not hydrophilic, and does not comprise hydrophilic functional groups, such as hydroxyl groups. Preferably, the organic polymer is not a naturally polymer, such as cellulose or starch. Such hydrophilic organic polymers are typically processed from aqueous solutions, and thus they cannot be moulded conveniently.

[0086] The composite material can comprise at least 1 wt.%, preferably at least 5 wt.% biochar, depending on the intended use. However, the environmental footprint can be especially advantageous when high amounts of biochar are included. For effective carbon storage, it is preferred that the composite material comprises a significant amount of biochar, preferably at least 20 wt.%, more preferably at least 30 wt.%, or even at least 40 wt.%. Preferably, the composite material comprises up to 60 wt.%, more preferably up to 80 wt.%, or even up to 90 wt.% biochar. It is preferred that the amount of biochar in the composite is from 10 wt.% to 90 wt.%, or from 20 wt.% to 80 wt.%. Preferably, the remainder in these embodiments is organic polymers, and optionally up to 10 wt.% additives.

[0087] In a preferred embodiment, the composite material comprises

[0088] (A) 10% to 90% organic polymer, and

[0089] (B) 10% to 90% modified biochar, wherein the amounts are in weight %, based on the total amount of solids. More preferably, the composite material comprises 20 to 80 wt.% biochar and 80 to 20 wt.% polymer.

[0090] In the above embodiments, the composite material may consist of the biochar and organic polymers. In another embodiment, the composite material may comprise additives (C), for example up to 20 wt.%, up to 10 wt.%, or up to 5 wt.%. Preferably, the total amount of components (A), (B) and (C) is 100%.

[0091] Subject of the invention is also a shaped object, which comprises or consists of the composite material of the invention as outlined above. A shaped object is a three- dimensional object (body, part), which has a defined form. The shape is conferred to the object deliberately in the production process. Thus, the shape is not random. For example, the shaped object can be a brick or panel. Preferably, the shape is conferred to the object by a moulding process. Alternatively, the shape can be conferred to the object after moulding, for example by cutting the moulded product. Subject of the invention is also a method for producing a composite material and / or shaped object of the invention, comprising the steps

[0092] (a) providing a composition comprising the modified biochar and the polymer or a precursor of the polymer,

[0093] (b) moulding the composition, and

[0094] (c) consolidating the moulded composition.

[0095] Typically, the composition of step (a) is intimately mixed, for example in a static mixer or extruder. The composition can be a liquid or paste. The viscosity of the composition can be adjusted by heating and / or additives. Preferably, the composition does not comprise solvent, such as water.

[0096] In step (b), the composition is moulded. The composition is filled into the moulding form (the mould), and is typically subjected to heat and pressure. Preferably, the temperature for compounding and / or moulding is between 120°C and 200°C, specifically between 140°C and 180°C. In the moulding form, the composition is consolidated at least in part. After partial or complete consolidation, the moulded part can be removed from the mould. Devices and methods for compounding and moulding polymers are commercially available and known in the art.

[0097] In step (c), the moulded composition is consolidated, thereby becoming the composite material and / or shaped object. The shape is conferred to the product by the moulding form. Depending on the composition, the product is dried and / or cured.

[0098] In a specific embodiment, the final shape can is conferred to the shaped object by further processing after moulding, such as cutting. Preferably, a plurality of objects is prepared, which all have the same shape and are suitable for large scale applications. Preferably, the process is continuous, which is advantageous for large scale application.

[0099] It is advantageous that the shaped objects can be prepared easily, uniformly and in large numbers by moulding. The shaped object has a defined three-dimensional form. Typically, the length in one direction is at least 5 cm, more preferably at least 10 cm or at least 20 cm, and / or the weight is at least 50 g, preferably at least 100 g. For example, the shaped object can have a length between 5 cm and 2 m, and / or a weight between 50 g and 10 kg. After moulding, the shape can be modified, for example by cutting or polishing. Preferably, the shaped object does not have a random structure. For example, the shaped object can be a brick, rod, plate, pellet, or object with a defined, more elaborate structure.

[0100] In a preferred embodiment, the shaped object is not porous. This means that the polymer matrix does not comprise pores or voids. However, the biochar particles typically comprise micropores. Such a dense material can be advantageous, because it stores a high amount of carbon per volume, whereas the stability and barrier properties can be high.

[0101] In a preferred embodiment, the composite material and shaped object can comprise at least one additional filler or reinforcement, which is different from the modified biochar. For example, the shaped object may comprise fibers. It is preferred that additional filler or reinforcing agent is also based on natural materials, and thus has a good environmental footprint. Additional fillers could be added for modifying the properties, for example by including colour pigments or conductive particles. Fibres, especially glass or carbon fibres, can be added for increasing and modifying mechanical stability. Preferably, the amount of additional fillers and / or reinforcing agents is less than 20 wt.%, preferably less than 10 wt.%, for example in the range of 1 to 20 wt.% or 2 to 10 wt.%.

[0102] The composite material and shaped object may comprise at least one additive, which is not a structural polymer, a filler or reinforcing agent. For example, the additive can be selected from processing aids, such as lubricants, plasticizers, colorants, flame retardants, thermal stabilizers and compatibilizers. Lubricants can improve workability during compounding or moulding. For example, the processing aid can be a fatty acid salt, such as zinc or calcium stearate, which is for example available under the trademark Ligastar from Peter Greven, DE. Such additives can also confer desired properties to the composite material and shaped object, such as colour or stability. Preferably, the amount of additives is up to 5 wt.% or up to 2 wt.%, for example in the range of 0.01 to 5 wt.% or 0.1 to 2 wt.%.

[0103] In a preferred embodiment, the composite material comprises a compatibilizer, for example in an amount of 0.1 to 8 wt.%, preferably 0.5 to 5 wt.%. A compatibilizer can improve the miscibility of the polymer blend with the filler, thereby increasing the stability of the composite material. Preferably, the compatibilizer is a derivative of the matrix polymer, which is functionalized with hydrophilic groups. Preferably, the hydrophilic group is a carboxylic group, especially a maleate group. Preferably, the matrix polymer is polyethylene and the compatibilizer is maleated polyethylene. It was found that such a maleated polymer can further increase the mechanical stability of the composite material comprising the modified biochar.

[0104] Preferably, the shaped object is rigid. Accordingly, it is preferably stiff, and thus has a low elasticity. Preferably, the tensile modulus is at least 4000 N / mm2, more preferably at least 5000 N / mm2, preferably between 4000 and 8000 N / mm2, as determined according to ISO 527-1 :2019. Rigid materials can be advantageous for various applications, such as construction applications, because of their high dimensional stability. Preferably, the tensile strength is high, for example >20 MPa, preferably >22 MPa, or even >25 MPa, as determined according to ISO 527-1 :2019. The high tensile strength is advantageous for building applications, because the material can withstand mechanical stress and strain.

[0105] According to the invention, it was found that the shaped objects can have high stability, even when high ratios of biochar are included. Thus, the shaped objects are highly suitable for construction applications, which enable permanent and efficient carbon storage. Without being bound to theory, it is assumed that the surface modification of the biochar can advantageously promote the compatibility with organic polymers, which allows intimate mixing and uniform dispersion.

[0106] Subject of the invention is also a construction part, which comprises or consists of the composite material of the invention. Subject of the invention is also the use of the modified biochar of the invention for producing construction parts. Preferably, the construction part is a panel, insulation board, building part or building block.

[0107] Construction applications are especially constructional engineering and architecture. They comprise building, but also infrastructure applications, such as bridges or tunnels. Preferably, the construction part or shaped object is a panel, insulation board, building part or building block. In preferred embodiments, the building material is a panel for walls, an insulation board, a building part, or a block for assembling building parts, such as walls, or a structural part. A panel is a flat object for covering a building part, such as a wall, floor or furniture surface. The use as a panel is advantageous because of the high stability of the composite material. Since the composite material can be mechanically, thermally and chemically stable, the panel can shield the substrate to which it is mounted. The construction part may comprise at least one further component or material. For example, it can be a laminate from two, three or more layers, which comprises a composite material and / or shaped object of the invention. In a preferred embodiment, the shaped object comprises a coating. For example, a functional coating can confer a desired property to the shaped object. The coating may confer colour and / or texture to the surface. The coating could be a protective coating, for example against moisture, UV radiation, mechanical damage or weathering. Since biochar confers a dark colour to the composite material, a coloured coating may be desired. Coatings can be applied by conventional means, such as liquid coating, for example with resins, impregnation, electron scattering (T respa process), physical or chemical vapor deposition, lamination and the like. The coating may cover the shaped object completely or partially, for example only on one surface.

[0108] The modified biochar of the invention can also be used for purposes, which are different from preparing polymer composites. In this regard, the modified biochar can be used for all applications, in which surface activation is desirable. For example, the modified biochar can be used as an adsorptive, for example for soil or water cleaning, as a catalyst or filter material.

[0109] The inventive methods, composite materials, shaped objects and construction parts solve the problem underlying the invention. A stable composite material is provided, which can be used for various applications. Shaped objects are especially suitable for construction applications which allow long-term storage of carbon from the atmosphere. Even when comprising relatively high amounts of biochar, the shaped objects can have high mechanical and dimensional stability.

[0110] By developing a process that utilizes maleic anhydride to exfoliate and “break down” larger biochar particles while simultaneously grafting to the surface via cycloaddition in a Diels- Alder reaction mechanism, a platform for the efficient modification of biochar surface chemistry and structure is provided, allowing it to be tailored for a specific polymer matrix.

[0111] This chemical surface modification of biochar helps to promote an even filler distribution of biochar and enhance adhesion to the polymer matrix, and also improves the handling and processability of biochar. By incorporating this chemically modified biochar in a polymer matrix with a matching chemical profile, a composite material can be produced that displays significant improvements in mechanical strength over other biochar-based composites. Further, the usage of surface functionalized char with a reduced water uptake, greatly increases biochar’s applicability as a filler in plastic materials, by allowing for combination with polymeric materials that are highly susceptibility to hydrolytic degradation during compounding.

[0112] The nanostructured biochar can advantageously be used for applications, for which the benefit of nanoscale morphologies has been reported for other materials, for example as an additive in a composite polymer matrix.

[0113] Figure 1 shows FT-IR spectra of the untreated spruce wood biochar substrate (upper dotted line) and inventive maleic anhydride modified spruce wood biochar (lower black line), as described in example 27.

[0114] Figure 2 shows the result of the FT-IR analysis of inventive maleic anhydride modified biochar with baseline correction, as described in example 27.

[0115] Figure 3 shows the result of TGA of mass loss vs. temperature for untreated biochar substrate (upper dotted line) and inventive maleic anhydride modified biochar (lower black line), as described in example 28.

[0116] Examples

[0117] Examples 1 to 16 (comparative)

[0118] In a first series of experiments, it was tried to modify the surface of wood biochar by reactions which have been described in the art for carbonaceous materials such as graphene, carbon black or activated carbon, which have relatively uniform structures and consist predominantly of carbon. In examples 1 to 8, the reactant was maleic anhydride (99%, Sigma). Further, it was tried to modify the biochar unspecifically under harsh conditions, with strong acid, strong base or hydrogen peroxide, or by oxidation with air at 400°C (examples 9 to 16). The substrate was biochar powder from spruce wood, which was produced at a high pyrolysis temperature of 650 - 850°C, and is relatively hard and inert. The particle diameters were in the range of 100 to 300 pm or 100 to 500 pm, as determined by sieve analysis. For comparison, graphite (325 mesh flakes, Sigma Aldrich) was used as a substrate in example 8.

[0119] The substrates, agents, process conditions, results, and documents which describe corresponding reactions, are summarized in table 1. In the reflux reactions, the reactants for each experiment are introduced in a round-bottom flask along with a magnetic stirrer and submerged in a heated oil bath. The flask is connected to a condenser to form a reflux setup for the duration of the reaction. If applicable, the modified biochar is filtered and washed with toluene. Subsequently, the presence of oxygen containing groups on the biochar surface was determined by Fourier Transform Infrared Spectroscopy (FTIR.

[0120] The result was that all experiments 1 to 16 failed. Oxygen groups, such as carboxyl or hydroxyl groups, could not be detected in any of the biochar products. Also graphite could not be modified with maleic anhydride in experiment 8. This suggests that even the modification of highly ordered graphite is more problematic than reported in the art.

[0121] The results demonstrate that reactions for surface modification, which were reported in the art for uniform and ordered carbon substrates, are not applicable for biochar. It was not even possible to modify the biochar unspecifically with harsh chemicals, such as sodium hydroxide, sulfuric acid or hydrogen peroxide (examples 9 to 15), or with air at 400°C (example 16). The results demonstrate clearly that it is difficult to modify the hard, chemically inert biochar. The results also confirm the finding from the prior art, that no method was so far available for modifying biochar with maleic anhydride.

[0122] Table 1: Conditions, result and references for comparative examples 1 to 16.

[0123]

[0124] Abbreviations: BC = biochar, MA = maleic anhydride, PSD = Particle size distribution, RT = room temperature, US = Ultrasound bath

[0125] Examples 17 to 26:

[0126] Based on the findings of examples 1 to 16, the reaction conditions of the reflux one-pot reactions were further modified with maleic anhydride as the reactant. The selected reactants for each experiment are introduced in a round-bottom flask along with a magnetic stirrer and submerged in a heated oil bath. The flask is connected to a condenser to form a reflux setup for the duration of the reaction. The modified biochar is then filtered and washed with toluene and tested with via FT-IR for the presence of functional groups associated with the presence of maleic anhydride. Several reaction parameters were varied, including temperature, reaction time, substrate, solvent, and material ratios. The efficacy of the reactions was monitored by FT-IR, in which the resulting spectra were examined, specifically for the presence of carboxyl groups on the substrate’s surface. The substrate, agents, conditions and results are summarized in table 2.

[0127] Table 2: Conditions and result for examples 17 to 26. Examples 17 to 21 are comparative and examples 22 to 26 are inventive. The ratio S:MA is substrate : maleic anhydride.

[0128] Surprisingly, it was found that the reaction with biochar was effective in examples 22 to 26, when the substrate was biochar and the temperature was significantly increased. In the modified biochar product, carboxyl groups could clearly be detected on the surface by IR spectroscopy. Notably, no modification was observed under the same conditions at 180°C with graphite, which comprises highly ordered graphene structures and is highly uniform. This confirms that the result with biochar is not predictable based on data with other carbonaceous substrates.

[0129] Example 27: FTIR analysis of reaction product

[0130] As noted above, in examples 1 to 26 the presence of functional groups on the substrates surface was analyzed with Fourier Transform Infrared Spectroscopy (FTIR; device Perkin Elmer LIATR 2). In Fig. 1 , the FT-IR spectra are shown for the raw biochar substrate from spruce wood char, and for the maleic anhydride modified spruce wood biochar of example 26. To reduce artefacts and identify the chemical structure of the modified biochar more precisely, a baseline correction was performed for the data shown in Fig. 1. The result is shown in Fig. 2.

[0131] As shown in Fig. 1 , no absorbance peaks are identified in the FT-IR spectrum of the untreated biochar substrate, indicating a lack of surface functionality. This can be explained by the high pyrolysis temperature, which is known to confer chemical inertness to biochar.

[0132] In contrast, as shown in Fig. 1 , the modified biochar has a series of absorbance bands in the region associated with the presence of carbon-oxygen bonds. The peaks are distinguished more clearly in Fig. 2. A strong C=O peak is present at 1719 cm-1, a C-0 peak at 1159 cm-1, and weaker peaks are observed in the fingerprint region. The C=O stretch at 1719 cm-1is distinctive for carboxyl groups. The weaker peaks in the fingerprint region are representative of maleic anhydride. Notably, no significant peaks are detected in the range between 2000 and 4000 cm-1, which shows that the inventive method is highly selective.

[0133] To confirm that the maleic anhydride was indeed grafted to the biochar surface, and not just coated onto the biochar, washing in toluene was carried out repeatedly to remove any non- covalently attached material. Additionally, heating the modified biochar in an oven to 200 °C, well above the temperature at which maleic anhydride is stable as a solid material, provided additional confirmation of the successful covalent grafting. The finding that the surface functionality on the biochar was only detected when the method was carried out at high temperature at 180°C, also confirms that the biochar is covalently modified, because a coating process would be unlikely to show such a temperature dependency. The FTIR spectra provide evidence that carboxyl groups were grafted selectively and in significant amounts onto the biochar surface. The introduction of the functional groups was especially surprising given the failure of experiments to yield any measurable alteration in the biochar surface chemistry, when the biochar was treated with strong acids or oxidizing agents, or when graphite was treated at 180°C.

[0134] Example 28: Thermogravimetric analysis (TGA)

[0135] The thermal stability of biochar before and after modification was measured by thermogravimetric analysis, in which the mass of a sample is measured over time as the temperature changes. For a TGA device (TGA2, Mettler Toledo, DE) a basic temperature ramp was used, where the samples were heated from 25°C to 800°C at a rate of 10°C / min in the presence of air. The TGA data of mass loss vs. temperature for the untreated biochar (upper dotted line) and modified biochar (lower black line) is shown in Fig. 3.

[0136] The results show reduced thermal stability of the modified biochar compared to the untreated biochar. The lower stability could be because of the chemical modification of the biochar by the reversible Diels-Alder reaction, and also the morphological transition to smaller particles.

[0137] FTIR analysis of the modified material after heating to 200°C for three hours also confirmed the presence of carboxyl groups, indicating the thermal stability of the formed adduct, and that a portion of the observed mass loss above 200°C stems from the grafted material on the surface of the biochar particles. Further, given that the degradation temperature of maleic anhydride is approximately 150-180°C, the results provide further evidence of the successful covalent modification of the biochar.

[0138] The TGA data confirms that the surface of the modified biochar comprises grafted carboxyl groups which are present at high density.

[0139] Example 29: Nanostructure of modified biochar

[0140] Unexpectedly, it was observed that the optical properties of the modified biochar powder were significantly different from the biochar substrate. While the biochar substrate powder was black, the modified biochar powder was brown, having approximately the colour of coffee powder. Such a colour change could indicate the formation of nanostructures. To confirm this, it was tested if the modified biochar was fluorescent under UV light. A water dispersion of the raw biochar substrate does not display fluorescence. However, UV illumination of a water dispersion of the modified biochar powder displays strong fluorescence. The development of fluorescence is indicative of an energy transfer through the particles via conjugation, which is normally only observed in nanoscale sized particles. Without being bound to theory, it is likely that the doping with oxygen, as a result of the cycloaddition reaction with maleic anhydride, influences the electronic structure of the nanoparticles, and therefore the band gaps between energy levels, due to the differing electronegativity of carbon and hydrogen and ion pairs of electrons. Overall, the optical analysis indicates that the modified biochar is nanostructured

[0141] Example 30: Preparation of composite materials

[0142] Composite materials were prepared from HDPE as a matrix polymer and modified biochar as prepared in example 26 above with the modification that the ratio of biochar to maleic anhydride was 1 :1. For comparison, corresponding materials were prepared with conventional biochar. The materials used are shown in table 3 and the compositions in table 4.

[0143] Table 3: Materials used for compounding

[0144] The modified biochar was added to the HDPE and additives at a temperature of 170°C in a rheometer (Plasti-Corder™, Brabender, DE) torque with mixer attachment rotating at 60 rpm. The compounded material was then pressed into a rectangular plate using compression moulding. After consolidation, samples for mechanical testing were manufactured using a CNC machine.

[0145] Examples 31 to 34: Examination of tensile properties

[0146] The tensile properties of the composite materials of examples 31 to 34 were tested. Tensile strength, tensile modulus and elongation at break were tested according to ISO 527:2019 with a Quasar 25 Galdabini UTM.

[0147] Table 4: Compositions and tensile properties

[0148] The results show that tensile properties of compounds comprising modified biochar as a filler are improved compared to corresponding compounds which comprise conventional biochar. The tensile strength and the elongation at break are significantly higher. The results suggest that the inventive compounds with modified biochar are more suitable for uses in which high mechanical stability is required, such as building applications.

[0149] The compound with modified char (example 33) exhibits an even higher increase in tensile strength and elongation at break than comparative compounds which comprise conventional biochar and MAgPE (example 32). This suggests that the filler surface maleate functionality is more effective in improving filler-matrix compatibility, and therefore filler dispersion, than the maleated polymer additive. This is coherent, since particles with individually covered surfaces have a higher surface area of modification in contact with the matrix; and therefore homogeneity is enhanced. Example 34 shows that the addition of maleate functionality to both the HDPE and the biochar can cause a very noticeable increase in compatibility, positively impacting tensile properties. It allows an increase in strength and elongation, without compromising stiffness (modulus). The compound had the highest tensile values, with an increase of up to 25% in tensile strength, 27% in elongation and 3% in modulus in comparison to the compound with conventional biochar of example 31 .

[0150] MA in different forms (ways of interacting with the polymer composite) were evaluated. In the form of maleated polyethylene (MAgPE), it showed an increase in tensile strength and elongation. The increased compatibility between char and PE with just a small addition (3%) of MAgPE, we believe, allowed for better char dispersion in compounding, and therefore can be reflected in its tensile properties. Allowing better stress dispersion within the sample, and better polymeric chain rotation in order to improve in elongation.

[0151] Example 35: Characterization of surface composition

[0152] The surface chemical modification of modified sprucewood biochar of example 26 above was examined by XPS before and after the modification reaction. The results in table 5 show that the biochar exhibits a significantly higher oxygen content after the modification reaction (-350% increase in O / C ratio), indicating MA functionalization on the surface. Because of the oven treatment at 260°C, which is well above MA’s boiling point, it can be concluded that the MA functionality on the modified char’s surface is grafted, and therefore possesses higher thermal stability.

[0153] Table 5: Surface composition determined by XPS

[0154] Example 36: Reaction Parameters vs Grafting Density

[0155] XPS was used to study if the ratio biochar to maleic anhydride (diene to dienophile, BC:MA) has an impact on the grafting density of MA on the biochar’s surface. The results in table 6 show that there is a strong correlation between the diene to dienophile ratio and grafting density, as seen from the 42% increase in O / C ratio from 1:1 to 1:4, and a 23% increase between 1:4 and 1 :10. It is concluded that good control on grafting density and surface O content can be achieved with the modification of the BC:MA ratio.

[0156] Table 6: Surface composition determined by XPS for different ratios BC:MA

[0157] Example 37 to 39

[0158] A composite was prepared from polycarbonate / acrylonitrile butadiene styrene (PC-ABS) and inventive modified biochar (example 39). For comparison, also a composite was prepared that comprised the respective biochar filler without surface modification (comparative example 38). Samples were created using a PC / ABS blend of 55 / 45 wt.%. Biochar was added to the polymeric material at a loading of 20 wt.%. The mechanical properties were examined. For comparison, also the properties of a composite without filler were examined. The results are shown in table 7. The results show that the overall mechanical properties of the composite with modified biochar are significantly improved compared to non-modified biochar.

[0159] Table 7: Mechanical properties of PC-ABS composites of example 37 Literature

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Claims

CLAIMS1. A method for preparing modified biochar, wherein biochar powder is modified in a chemical reaction with maleic anhydride, wherein the reaction is carried out at a temperature of at least 150°C, wherein the modified biochar is not subjected to carbonization after the reaction.

2. The method of claim 1 , wherein the reaction is a [4+2] cycloaddition (Diels-Alder reaction).

3. The method of at least one of the preceding claims, wherein the reaction product is hydrolyzed.

4. The method of at least one of the preceding claims, wherein the biochar is wood biochar.

5. The method of at least one of the preceding claims, wherein the biochar was prepared by pyrolysis at a temperature of more than 500°C and / or wherein the carbon content of the biochar is from 70 wt.% to 95 wt.%.

6. The method of at least one of the preceding claims, wherein the reaction mixture comprises, based on the total amount of solids,(a) 5% to 50 wt.% biochar, and(b) 50% to 95 wt.% maleic anhydride.

7. The method of at least one of the preceding claims, wherein the reaction mixture does not comprise water as a solvent.

8. The method of at least one of claims 1 to 6, wherein the reaction mixture does not comprise a solvent, or wherein the reaction mixture comprises a solvent which has a boiling point above 180°C, wherein the solvent is preferably ethylene glycol.

9. The method of at least one of the preceding claims, wherein the pH value of the reaction mixture is between 4 and 10.

10. A modified biochar, which is obtainable by a method of at least one of claims 1 to 9.11 . A modified biochar, which comprises functional groups selected from(1) and (2).

12. The modified biochar of at least one of claims 10 and 11 , which is nanostructured and / or fluorescent under UV light when dispersed in water.

13. The modified biochar of at least one of claims 10 to 12, wherein the O / C-ratio on the surface is at least 0.4, preferably at least 0.6, as determined by X-ray photoelectron spectroscopy (XPS).

14. A composite material comprising a polymer matrix, in which the modified biochar of at least of claims 10 to 13 is distributed, wherein the polymer is preferably a thermoplastic polymer.

15. A shaped object comprising the composite material of claim 14.

16. A construction part, which comprises a composite material of claim 14 or shaped object of claim 15, which is preferably a panel, insulation board, building part or building block.

17. A method for producing a composite material and / or shaped object of at least one of claims 14 to 15, comprising the steps of(a) providing a composition comprising the modified biochar and the polymer or a precursor of the polymer,(b) moulding the composition, and(c) consolidating the moulded composition.

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