Method for alkaline pulping of biomass

Abstract

Herein described is a method for alkaline pulping of biomass in an aqueous mixture, comprising a conversion step in which biomass comprising lignin is converted in an aqueous liquor to obtain converted biomass, wherein the aqueous liquor comprises a protective agent, a basic compound, water, and optional additional pulping reagents. Also described are lignin, xylan and cellulose obtainable by said method.

Description

[0001] November 10, 2025

[0002] Method for alkaline pulping of biomass

[0003] TECHNICAL FIELD

[0004] The invention relates to a method for alkaline pulping of biomass in an aqueous mixture, as well as to lignin and cellulose obtainable by said method.

[0005] BACKGROUND

[0006] Biomass obtained from plants typically comprises cellulose, hemicellulose, and lignin. Chemical pulping techniques, such as soda and Kraft processes, have played a pivotal role in cellulose extraction from wood chips for over a century. Generally, these processes subject wood chips to high pH at elevated temperatures to remove lignin from the lignin-carb ohydrate complexes. Despite satisfactory lignin removal efficiency, the harsh conditions of these established processes severely condense lignin by cleaving the native p-0-4 ether linkages of the lignin units and forming recalcitrant C-C bonds. Applications of the resulting heavily condensed lignin are limited, often ending in combustion for heat recovery.

[0007] Recent focus has shifted towards lignin as a promising source of renewable aromatics for sustainable chemical and material production. However, preserving the native structures of lignin - including the reactive p-0-4 ether linkages - during fractionation is essential for efficient depolymerization to produce valuable aromatic monomers and to enable subsequent functionalization of the lignin monomers, as degraded or highly condensed lignin cannot easily be depolymerized. Furthermore, a high degree of condensation also renders lignin chemically recalcitrant for material applications.

[0008] In response to the rising demand, so-called “lignin-first” biorefining methods have been developed, including reductive catalytic fractionation (RCF), aldehyde-assisted fractionation (AAF), and mild acidolysis with ethylene glycol stabilization, which can yield high-quality lignin or lignin-derived monomers. However, the cellulose resulting from these lignin-first methods cannot meet the requirements of the pulp and paper industry. The acidic conditions during AAF and mild acidolysis with ethylene glycol stabilization trigger cellulose hydrolysis, reducing the fiber length and strength of the resulting cellulose. RCF, in contrast, employs a precious metal-based catalyst, leading to metal contamination in the pulp.

[0009] Ribeiro et al. (ACS Sustainable Chem. Eng. 2019, 7, 10274-10282) describes a process wherein wood is subjected to the conventional Kraft pulping method, resulting in the extraction and degradation of approximately 90-95% of the lignin originally present in the wood. The study examines the residual lignin that remains in the pulp following Kraft processing. For analytical purposes, the residual pulp is enzymatically digested to remove carbohydrates, thereby isolating cellulolytic enzyme lignin (CEL). As noted on page 3, the purpose of this approach is to provide further insight into the influence of varying residual effective alkali on the functional groups present in pulp residual lignin. The characterized lignin is, therefore, the fraction that remains associated with the pulp and is not solubilized during the Kraft process. They further observe that both residual and dissolved Kraft lignin fractions exhibit a higher content of free phenolic groups compared to native wood lignin, with the dissolved fractions showing particularly elevated values. The proportion of p-0-4 linkages is also reported to be significantly lower in residual lignin, reaching only about 15%, relative to the levels observed in native lignin samples.

[0010] Additionally, all above-mentioned lignin-first approaches employ various organic solvents, which make them unsuitable for application in a large-scale industrial pulping process. Their operating conditions also significantly deviate from those of the conventional pulping process, which makes it challenging to adapt the existing plants to the novel technologies.

[0011] HT / rp 241043WO

[0012] 10 November 2025 Thus, it is an object of the present invention to provide a method that addresses these and other needs in the state of the art. Ideally, the undesirable lignin condensation can be reduced. Further, it is desirable to produce both high-quality cellulose and high- quality lignin in the same process, in particular using conditions compatible with industrial pulping processes.

[0013] Other and further objects, features, and advantages of the present invention will become apparent more fully from the following description.

[0014] SUMMARY OF THE INVENTION

[0015] Some or all of these objects are achieved with the present invention by the method according to claim 1, the lignin fragments according to claim 17, the lignin according to claim 19, the cellulose according to claim 20, and the xylan according to claim 21.

[0016] It was surprisingly found that the method according to the invention ensured the preservation of p-0-4 linkages, crucial for lignin depolymerization, and reduced condensation. At the same time, good lignin removal efficiencies could be achieved. This was achieved by employing a protective agent in the pulping process, which reacted with diol motifs present in lignin under the alkaline aqueous conditions of the method according to the invention.

[0017] Without wishing to be bound by scientific theory, it is believed that the protective agent reacts with diol motifs of the lignin structure under alkaline aqueous conditions to form an intermediate, as detailed in Scheme 1 using a p-0-4 motif and a boric or boronic acid as an illustrative and non-limiting example. In Scheme 1, “R” may signify various substituents, for example aryl or alkyl groups rendering the protective agent a boronic acid, or a hydroxyl group rendering the protective agent boric acid. The resultant intermediate stabilizes the p-0-4 ether linkages in the lignin structure; one example of such an intermediate is represented by the cyclic boric or boronic ester shown in Scheme 1. This stabilization is believed to significantly reduce lignin

[0018] HT / rp 241043WO

[0019] 10 November 2025 condensation during the pulping process, thereby presumably increasing the quality of the resultant lignin. It is further believed that through the use of the protective agent, the use of harsh pulping conditions is possible without resulting in severe lignin condensation.

[0020] Scheme 1: Generalized protection reaction of a p-0-4 motif in a lignin structure.

[0021] It was surprisingly found that the cyclic ester formed between the lignin p-0-4 diol and the boric or organoboronic acid can be easily hydrolyzed by acid to regenerate the free lignin p-0-4 diol structure as seen in native lignin. In particular, the borate ester formed between the lignin p-0-4 moiety and boric acid is rapidly hydrolysable at room temperature within minutes by an acidifying agent, such as H2SO4, heterogenous Bronsted acids, HC1 or CO2, preferably HC1 or CO2, even more preferably CO2.

[0022] Scheme 2: Generalized deprotection reaction of a BA-protected p-0-4 motif in an extracted lignin.

[0023] In contrast, in organic solvents and under acidic conditions of the AAF process, diol motifs would form acetals with aldehyde reagents. However, such protection would be unsuitable for industrial pulping as the reaction conditions of the industrial pulping process would result in immediate deprotection of the diol motifs. The method

[0024] HT / rp 241043WO

[0025] 10 November 2025 according to the invention, however, is tailored to be compatible with industrial pulping in terms of reaction conditions.

[0026] It was surprisingly found that the lignin resulting from the method according to the invention is of high quality, making it suitable for further depolymerization and / or functionalization. In particular, the resulting lignin can produce 4-5 times more lignin monomers upon reductive catalytic depolymerization. Compared to technical lignin, the physical and chemical properties of the resulting protected lignin can also be tuned by using different protective agents. It is further noticed that the resulting lignin may have a significantly lighter colour compared to technical lignin from traditional pulping processes. Also, the cellulose resulting from the method according to the invention was surprisingly also of high quality, meeting the industry requirements with regard to strength, fiber length, and bleachability, making it a good substitute to industrial pulp in its various applications, such as paper making to packaging.

[0027] A further advantage is that xylan, which is a main constituent of hemicellulose, is also preserved in its polymeric form during this pulping process. The resulting xylan can be easily hydrolyzed to produce xylose monomer. It can also be used in its polymeric form in material applications.

[0028] The term “biomass” may in particular refer to hardwood, such as beech, birch, and / or poplar; softwood, such as pine, spruce, and / or fir; herbaceous species, such as herbaceous bamboo, sorghum bagasse, sugarcane bagasse; and / or corn stover; and / or mixtures thereof. The biomass may be provided in any shape and / or size, such as in the form of powder, shave, dust, chips, and / or mixtures thereof.

[0029] The term “converted biomass” stands for biomass as defined above that has undergone conversion reaction by protecting the reactive p-0-4 linkages present in the lignin contained in the biomass.

[0030] PREFERRED EMBODIMENTS OF THE INVENTION

[0031] HT / rp 241043WO

[0032] 10 November 2025 The present invention provides a method for alkaline pulping of biomass in an aqueous mixture, comprising a conversion step in which biomass comprising lignin is converted in an aqueous liquor to obtain converted biomass. The aqueous liquor comprises i) a protective agent, ii) a basic compound, hi) water, and iv) optional additional pulping reagents. The method according to the invention surprisingly achieved high lignin removal efficiency without compromising lignin quality by employing a protective agent to protect the reactive p-0-4 linkages present in the lignin during the conversion step.

[0033] Further advantageous embodiments of the invention are specified in the dependent claims and are elucidated in detail herein below.

[0034] The conversion step of the method according to the invention is advantageously conducted at a temperature of 80 °C to 250 °C, preferably of 80 °C to 200 °C, more preferably 80 °C to 180 °C. The conversion step of the method according to the invention may also be conducted at a temperature of 80 °C to 250 °C, preferably of 100 °C to 200 °C, more preferably of 130 °C to 180 °C. At these temperatures, the lignin removal efficiency is good without compromising the quality of the resultant lignin. Lower temperatures result in no or very slow lignin removal, while very high temperatures can achieve high lignin removal efficiency, but lignin condensation is more severe at these extremely high temperatures. Nonetheless, more p-0-4 linkages are preserved with the method according to the invention compared to the methods of the prior art without the protective agent.

[0035] To mitigate lignin condensation, a shorter reaction time can be used. However, shortening the reaction time in turn decreases the lignin removal efficiency. In a preferred embodiment of the method according to the invention, the conversion step is conducted for a total duration of 1 h to 20 h, preferably of 1.5 h to 10 h, more preferably of 2 h to 8 h, even more preferably of 3 h to 6 h. At these reaction times, a good balance between lignin removal efficiency and lignin condensation is achieved.

[0036] HT / rp 241043WO

[0037] 10 November 2025 Preferably, the aqueous mixture has an alkaline pH. In a further embodiment of the method according to the invention, the pH of the aqueous mixture is 10 or higher, preferably 12 or higher, more preferably 13 or higher, even more preferably 13.5 or higher. At these pH values a good lignin removal efficiency can be achieved while avoiding severe lignin degradation. Under acidic conditions, the reaction between the P-0-4 motifs and the protective agent is hampered, while at mildly alkaline conditions the lignin removal efficiency is very low. At extremely high pH values, however, lignin degradation is severe.

[0038] In a preferred embodiment of the method according to the invention, the protective agent reacts with lignin to form a hydrolysable intermediate, in particular an ester intermediate, particularly preferably a boronic ester intermediate or a borate ester intermediate. This hydrolysable intermediate provides protection for the p-0-4 motifs present in the lignin and may also react with other diol motifs present in the biomass, such as those of xylose. After the pulping process is finished, the protective agent can be removed from the lignin by simple hydrolysis, leaving the p-0-4 motifs free to partake in any further desired reactions.

[0039] The protective agent may comprise or consist of a compound comprising a group 13 element, in particular boron. Preferably, the protective agent comprises or consists of a boric acid compound, in particular a boric acid compound selected from the group consisting of boric acid, alkali salts of boric acid, or mixtures thereof, and / or a boronic acid compound, in particular a boronic acid compound selected from the group consisting of optionally substituted phenylboronic acid, optionally substituted Ci to Ce-alkyl boronic acids, optionally substituted heterocyclic boronic acids, and mixtures thereof. Substituents may include, for example, alkyl groups, aryl groups, halogen atoms, alkoxy groups, alkyloxy groups, and / or mixtures thereof. Most preferably, the protective agent comprises or consists of phenylboronic acid, and / or boric acid, and / or alkali salts thereof, which has the advantages of being stable at room temperature and being readily commercially available. According to a particularly

[0040] HT / rp 241043WO

[0041] 10 November 2025 preferred embodiment, the protective agent comprises or consists of boric acid, alkali salts of boric acid, or mixtures thereof. One further advantage when using boric acid and / or alkali salts thereof as protective agents is that they can be removed fast and nearly quantitatively by simple acidification after extraction, affording native lignin.

[0042] According to one embodiment of the method according to the invention, the basic compound is soluble in water. Preferably, the basic compound is selected from the group consisting of alkali metal oxides, alkaline earth metal oxides, alkali metal hydroxides, alkaline earth metal hydroxides, ammonia, and mixtures thereof. The basic compound may be present in the aqueous liquor in a concentration of 0.01 to 10 M, preferably 0.1 to 5 M, more preferably 0.5 to 2 M. With such basic compounds employed in the given concentrations, it is possible to achieve a suitably alkaline aqueous mixture to allow for good lignin removal efficiency without compromising lignin quality.

[0043] In a preferred embodiment of the method according to the invention, the aqueous liquor is substantially free of organic solvents. Without wishing to be bound by scientific theory, lignin protected by phenylboronic acid or boric acid has very limited solubility in organic solvents such as 1,4-dioxane in the presence of basic compounds, but it is highly soluble in alkaline aqueous conditions. This can be explained by lignin being protected in the form of boric or boronic acid esters that can easily associate with a hydroxide anion from the alkaline cooking liquor. As such, the lignin protected by boric or boronic acids has a much higher charge density and, therefore, a much higher solubility in the cooking liquor. This high solubility significantly improves the lignin extraction efficiency, while the alkaline aqueous conditions are moreover compatible with industrial pulping conditions.

[0044] Advantageously, the aqueous mixture comprises the aqueous liquor and the biomass at a mass ratio aqueous liquor:biomass in the range from 1:1 to 40:1, preferably from 2:1 to 20:1, more preferably from 2:1 to 10:1, even more preferably from 3:1 to 8:1. A mass ratio in this range results in a good pulping efficiency.

[0045] HT / rp 241043WO

[0046] 10 November 2025 According to the method of the invention, the aqueous liquor comprises optional additional pulping reagents. Various pulping reagents are suitable for us in the method according to the invention as optional additional pulping reagents, as long as they do not react with the protecting agent, do not trigger lignin condensation beyond the protective effect of the boron species, and do not reduce lignin solubility to such an extent that lignin precipitates in the cooking liquor during extraction or postextraction processing. According to one embodiment of the method according to the invention, the optional additional pulping reagents are selected from the group comprising hydrogen sulfide, alkaline sulfide, alkaline bisulfide, sulfurous acid, alkaline sulfite, alkaline bisulfite, alkaline earth sulfite, alkaline earth bisulfite, ammonium sulfite, ammonium bisulfite, alkaline carbonate, alkaline bicarbonate, borohydride salts, surfactants, antifoaming agents and mixtures thereof, preferably selected from the group consisting of hydrogen sulfide, alkaline sulfide, alkaline bisulfide, sulfurous acid, alkaline sulfite, alkaline bisulfite, alkaline earth sulfite, alkaline earth bisulfite, ammonium sulfite, ammonium bisulfite, alkaline carbonate, alkaline bicarbonate, and mixtures thereof. These additional pulping reagents may improve the pulping efficiency. Examples of surfactants are sodium dodecyl sulfate, alkylbenzene sulfonates, alcohol ethoxylates, polyethylene glycol (PEG) derivatives, polyols and mixtures thereof. Chelating agents can preferably be selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitriloacetic acid, and mixtures thereof.

[0047] In a preferred embodiment of the method according to the invention, the aqueous liquor is premixed and combined with the biomass. It is, however, also possible to mix the biomass with, for example, the protective agent and add an aqueous solution comprising the basic compound, water, and optionally additional pulping reagents.

[0048] The aqueous liquor is preferably a solution.

[0049] HT / rp 241043WO

[0050] 10 November 2025 ln a further preferred embodiment of the method according to the invention, the pulping is conducted in a batch reactor or in a continuous reactor. A batch reactor has the advantage of being relatively easy to set up and offering higher flexibility, which is particularly suitable for smaller-scale productions. A continuous reactor offers higher efficiency, higher throughput, and better consistency across the process, and is particularly suitable for large-scale production. When a batch reactor is used, the reaction is preferably referred to as being conducted in batch mode.

[0051] Advantageously, the method comprises a separating step in which the converted biomass is separated from the aqueous mixture. The separated aqueous mixture may then be recycled and used in further conversion steps, which is particularly beneficial from an economic and environmental perspective. Before using the separated aqueous mixture in a further conversion step, lignin is preferably extracted from the separated aqueous mixture in a lignin extraction step.

[0052] In one embodiment of the method according to the invention, the method is performed in batch mode and the conversion step is repeated by combining the converted biomass with aqueous liquor for conversion at the same or a different temperature for the same or a different duration as the unconverted biomass. The aqueous liquor may be fresh or recycled aqueous liquor. The recycled aqueous liquor may have a residual lignin content from prior conversion steps. When the method comprises a separating step, conducting the conversion step and the separating step may also be referred to as one cycle. The cycles can be repeated one or several times. This so-called sequential fractionation allows to increase the lignin removal efficiency while avoiding exposing the extracted lignin to high temperatures for a long time. In a particularly preferred embodiment, the temperature is successively increased for each of the cycles. In this way, the remaining unreacted biomass can be subjected to higher temperatures with each cycle without affecting the lignin already extracted at lower temperatures, as this is removed after each cycle.

[0053] HT / rp 241043WO

[0054] 10 November 2025 - Il in another embodiment of the method according to the invention, the method is performed in a continuous mode, optionally using a counter-current reactor, crosscurrent reactor or mixed-current reactor. In a counter current reaction, the biomass and the aqueous liquor may flow in opposite directions, with the biomass continuously being fed on one side of the reactor and the aqueous liquor on the other side. In a cross-current reactor, the biomass usually moves continuously through the reactor, while fresh portions of aqueous liquor are added at several points along its path, typically at right angles to the flow of biomass. In a mixed current reactor, both the biomass and the aqueous liquor are thoroughly mixed together, resulting in a uniform composition throughout the reactor. As detailed above for the batch mode, the aqueous liquor may be fresh or recycled aqueous liquor, and the recycled aqueous liquor may have a residual lignin content.

[0055] Advantageously, lignin is extracted from the aqueous mixture in a lignin extraction step. The lignin extraction step is preferably conducted after the conversion step, more preferably after the separating step. In this way, the aqueous mixture after extraction may be recycled and used as aqueous liquor for another conversion step. In a preferred embodiment of the method according to the invention, the lignin extraction step comprises precipitation of lignin from the aqueous liquor. In this case, this step may also be referred to as a lignin precipitation step. If the extraction step comprises precipitation of lignin, it is most preferably conducted after the separating step. This precipitation may in particular be achieved through acidification preferably by acidification with HC1 and / or CO2, most preferably by acidification with CO2 or through use of an antisolvent or by a combination thereof. Particularly preferably, precipitation is achieved through use of an antisolvent. Without wishing to be bound by scientific theory, it is believed that acidified lignin has good solubility in many organic solvents. Lignin salt formed through antisolvent precipitation is only soluble in highly alkaline solvents, which may comprise water and inorganic and / or organic bases.

[0056] HT / rp 241043WO

[0057] 10 November 2025 Advantageously, lignin and xylan are co-extracted from the aqueous mixture in a lignin-xylan co-extraction step. The lignin-xylan co-extraction step is preferably conducted after the conversion step, more preferably after the separating step. In this way, the aqueous mixture after extraction may be recycled and used as aqueous liquor for another conversion step.

[0058] In a preferred embodiment of the method according to the invention, the lignin-xylan- co-extraction step comprises the co-precipitation of lignin and xylan from the aqueous liquor to obtain a lignin-xylan co-precipitate. In this case, this step may also be referred to as a lignin-xylan co-precipitation step. If the extraction step comprises coprecipitation of lignin and xylan, it is most preferably conducted after the separating step. This precipitation may in particular be achieved through acidification or through use of an antisolvent or by a combination thereof, preferably by acidification with HC1 and / or CO2, most preferably by acidification with CO2. CO2 being advantageous because of its low cost.

[0059] In another preferred embodiment of the invention, the lignin-xylan co-precipitate is separated into a lignin containing fraction and a xylan containing fraction. Preferably this is achieved by the addition of an organic solvent, more preferably by the addition of an organic solvent selected from the group consisting of acetone, ethanol, isopropanol, 1,4-dioxane, 2-methyltetrahydrofuran and ethyl acetate. Thereby lignin can be solubilized while xylan remains in its solid form. Thereafter the fractions can be separated preferably by filtration and / or centrifugation, more preferably by filtration.

[0060] Another aspect of the present invention relates to lignin fragments comprising repeating units of the general formula 1 and / or repeating units of the general formula 11

[0061] HT / rp 241043WO

[0062] 10 November 2025 wherein

[0063] R1is selected from the group consisting of hydroxy, methyl, ethyl, propyl, butyl, phenyl, p-tolyl, p-fluorophenyl, p-chlorophenyl, p-aminophenyl, thiophen-2-yl, 2- pyridyl, cyclohexyl, and benzyl and the methoxy groups shown in parentheses may be present or absent, and wherein repeating unit 1 can be present in a neutral or anionic form and wherein the lignin fragment has a content of p-0-4 linkages of 20% to 75 % relative to that of native lignin.

[0064] Within the context of the present invention, the term lignin fragment (also called lignin) stands for lignin fragments that underwent the conversion step according to the present invention, optionally followed by a separation step and, optionally, by acidification. Thus, with the method according to the present invention the lignin fragments retain at least 20% to 75%, preferably at least 25% and even more preferably at least 30% of the p-0-4 linkages compared to the amount present in native lignin (i.e. the form of lignin that exists in its original, unaltered structure and state within the cell walls of lignocellulosic biomass).

[0065] The term “repeating unit 1 can be present in a neutral or anionic form” means that the boron atom within repeating unit 1 can either have no net charge (neutral form as shown in the figure above) or, under alkaline pH conditions, attach to a hydroxyl (OH) group, resulting in a negatively charged (anionic) form.

[0066] HT / rp 241043WO

[0067] 10 November 2025 In addition to the repeating units I and II, the lignin fragments can comprise further repeating units that are naturally occurring in lignin that are formed, for example, via P~P or 5-5 linkages. Typically, 45 to 80 % of all monomer units in the lignin fragments are either repeating units I or II (the amount of the repeating unit II depends on whether an acidification took place or not). Additional repeating units may include various structures formed between coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol, such as p~p or 5-5 linkages. The exact composition depends on the specific type of lignin. Different plant species and their respective lignin types can have varied proportions of repeating units and linkages, resulting in distinct lignin structures and properties. During conventional lignin extraction, typically under acidic and elevatedtemperature conditions, the p-0-4 linkages, which are the most abundant and reactive bonds in native lignin, are highly prone to cleavage. This cleavage leads to the formation of a reactive benzylic carbocation intermediate. Once formed, this intermediate can readily undergo electrophilic aromatic substitution with other lignin units, resulting in undesired carbon-carbon (C-C) condensation reactions. These reactions form stable linkages such as p-5 or 5-5 bonds, which are not present in high abundance in native lignin but accumulate during processing. The result is a highly condensed, poorly soluble, and chemically less accessible lignin structure, which poses major challenges for downstream valorization. This is not the case due to the process according to the present invention which stabilizes the a-position and prevents the formation of the reactive benzylic carbocation in the first place. As a result, the p-0-4 linkages remain intact, and undesired condensation reactions are largely avoided. However, naturally occurring carbon-carbon linkages in lignin, such as p~p and 5-5 bonds, are not affected by the method according to the present invention. These linkages are inherently more stable and do not react with the protective agent under the conditions used in the conversion step. Therefore, there is no need to specify the “further repeating units that are naturally occurring in lignin and are connected for example via p~p or 5-5 linkages”, as they are neither affected by the protection step nor by the optionally following deprotection step. Thus, a typical composition of lignin fragments according to the present invention consists of 45 to 80% repeating units I

[0068] HT / rp 241043WO

[0069] 10 November 2025 and II, preferably 60 to 75 %, while the remainder comprises other repeating units that are naturally occurring in lignin and differ from repeating units I and II.

[0070] During the optional acidification step, the amount of repeating unit I is reduced, while the amount of repeating unit II is increased.

[0071] A preferred embodiment is directed to lignin fragments in which repeating units I and II are present, with repeating unit I accounting for 0.1% to 10% of their total quantity (i.e. repeating units I and II), and repeating unit II accounting for 90% to 99.9%. In another preferred embodiment, among the repeating units 1 and 11, only repeating units II are present, indicating that all repeating units I have been fully hydrolyzed.

[0072] The method according to the present invention enables the production of a very stable intermediate, namely lignin fragments with a high amount of repeating units of the general formula I which stabilize the p-0-4 linkages in the lignin structure. This stabilization is believed to significantly reduce lignin condensation during the pulping process, thereby presumably increasing the quality of the resultant lignin. Especially good results could be obtained for lignin fragments comprising repeating units of the general formula I, wherein R is hydroxyl or phenyl. The cyclic ester formed between the lignin p-0-4 diol and the boric or organoboronic acid of the repeating unit I is hydrolysable by acid to regenerate the free lignin p-0-4 diol structure as seen in native lignin, thus, resulting in lignin fragments comprising mainly repeating units of the general formula II. Thus, this simple deprotection method provides a straightforward method to recover the native hydroxyl groups in lignin. The remaining boric acid in lignin persists through lignin hydrogenolysis, largely because its simple, inorganic nature makes it resistant to various reactive environments. The present invention is furthermore directed to lignin obtainable by the method according to the invention. This lignin has a total hydroxyl group content of 1 mmol / g to 10 mmol / g, preferably of 1.5 mmol / g to 5 mmol / g, and / or a weight-average molecular weight of 1000 g / mol to 30000 g / mol, preferably of 2000 g / mol to 20000 g / mol, and a content of p-0-4 linkages of at least 20%, preferably 25% to 75%, relative to that of native lignin, and / or a polydispersity index of 2 to 40, preferably of 2 to 20. Lignin (i.e. lignin fragments) with a high hydroxyl group content and a

[0073] HT / rp 241043WO

[0074] 10 November 2025 content of p-0-4 linkages of at least 20% relative to that of native lignin allow for the development of materials with outstanding physical and chemical properties, including greater strength, improved elasticity, and superior thermal resistance. In addition, these lignin fragments help optimize processing by improving the viscosity and flow characteristics of the materials, which in turn enhances production efficiency. Their high hydroxyl content further promotes the creation of denser and more robust polymer networks, leading to products with increased durability and stability. Further, a p-0-4 content of at least 20% relative to that of native lignin after extraction indicates that the lignin is largely uncondensed, has undergone little structural degradation, remains close to its native state, and is therefore particularly suitable for further applications.

[0075] The present invention is also directed to cellulose obtainable by the method according to the invention. This cellulose has a residual lignin content of 0.5 wt% to 20 wt%, preferably of 1 wt.% to 20 wt.%, more preferably of 1 wt% to 10 wt%, even more preferably of 3 wt.% to 10 wt.%, and / or a mean fiber length of at least 1.1 mm. The low lignin content of the cellulose allows for good bleachability, and the long fibers contribute to excellent mechanical strength.

[0076] The present invention is also directed to xylan obtainable by the method according to the invention. This xylan typically has a high purity of greater than 90%, and more preferably greater than 95%, although a lower purity can also be obtained if desired by employing less rigorous washing.

[0077] Some exemplary embodiments of the invention are described in more detail with the aid of the schemes depicted in Figures la, lb, 1c, 2a, 2b, 2c, 2d, 2e, 2f, 2g, and 2h.

[0078] They show:

[0079] Figure la: a scheme of a process in which multiple batch-mode reaction cycles are combined to simulate a continuous cross-current extraction process.

[0080] HT / rp 241043WO

[0081] 10 November 2025 Figure lb: a scheme of a process in which multiple batch-mode reaction cycles are combined to simulate a continuous mixed-current extraction process.

[0082] Figure 1c: a scheme of a process in which multiple batch-mode reaction cycles are combined to simulate a continuous counter-current extraction process.

[0083] In Figure la a scheme of process in which multiple reaction cycles are combined to simulate a continuous cross-current extraction process is shown. Herein untreated wood chips (UWC) are fed into cycle SI, fresh liquor (FL) is fed into every cycle S1-S4 and the filtered solids (FS) of the respective preceding cycle are fed into Cycles S2 to S4. The product liquors L1-L4 are obtained from each of their respective cycles. The final solid fraction (Pl) is obtained from cycle S4.

[0084] In Figure lb a scheme of process in which multiple batch-mode reaction cycles are combined to simulate a continuous mixed-current extraction process is shown. Herein untreated wood chips (UWC) are fed into cycles SI, fresh liquor (FL) is fed into every cycle SI and the cycles S1-S4 as depicted, the separated liquors (SL) of cycles S3 and S4 are fed into successive cycles as depicted, and the filtered solids (FS) of the respective preceding cycle are fed into Cycles S2 to S4. The product liquors LI are obtained from each cycle SI and the product liquors L2-L5 are obtained from their respective cycles (S2) as depicted. The final solid fractions (Pl and P2) are obtained from the respective cycles S4.

[0085] In Figure lb a scheme of process in which multiple batch-mode reaction cycles are combined to simulate a continuous counter-current extraction process is shown. Herein untreated wood chips (UWC) are fed into cycle SI, fresh liquor (FL) is fed into every cycle S4 and the first generations of cycles (S1-S4, top row), the separated liquors (SL) of cycles S2-S4 are fed into successive cycles as depicted, and the filtered solids (FS) of the respective preceding cycle are fed into Cycles S2 to S4. The product liquors L1-L5 are obtained from the respective cycle SI. The final solid fractions (Pl and P2) are obtained from the respective cycles S4.

[0086] HT / rp 241043WO

[0087] 10 November 2025 Figures 2a to 2h show different NMR spectra:11B NMR spectra of (a) boric acid in 0.1M NaOD, (b) never acidified lignin, (c) 2,4-pentanediol and boric acid model reaction in 0.1M NaOD at 60°C for 1 h, (d) once acid-washed lignin and (e) twice acid- washed lignin.11B peaks assigned according to the literature report.1H NMR spectra of (f) never acidified lignin, (g) once acid-washed lignin and (h) twice acid-washed lignin.1H peaks assigned according to literature reports (Shuai, L. et al. Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization. Science (80-. ). 354, 329-333 (2016); Kim, H. & Ralph, J. A gel-state 2D-NMR method for plant cell wall profiling and analysis: A model study with the amorphous cellulose and xylan from ball-milled cotton linters. RSC Adv. 4, 7549-7560 (2014), Van Duin, M., Peters, J. A., Kieboom, A. P. G. & Van Bekkum, H. Studies on borate esters, Tetrahedron 40, 2901-2911 (1984)). Each stack of spectra, except the insets, was plotted with the same scaling factor. Lignin samples of similar masses were loaded for the analysis.

[0088] In the following, the invention is further described by way of examples that are in no way meant to be limiting.

[0089] EXAMPLES

[0090] Materials

[0091] Hydrochloric acid (37 wt%, Merck), sulfuric acid (95-97 wt%, Supelco), ruthenium on carbon (extent of labeling: 5 wt% Ru, Sigma-Aldrich) , 1,4-dioxane (>99.5%, Carl Roth), phenylboronic acid (99 wt%, Sigma-Aldrich), boric acid (98 wt%, Carl Roth), NaOH (IM, Sigma-Aldrich), n-decane (standard grade, TCI Europe NV), Dimethylsulfoxide-d6 (DMSO-d6, >99.9 atom% D, Cambridge Isotope Laboratories) Birch wood, beech wood, and pine wood were harvested in Switzerland and air-dried.

[0092] Analytical Methods

[0093] Determination of lignin content in cellulose

[0094] The lignin content in cellulose was measured as Klason lignin by acid digestion of a cellulose sample. The digestion was conducted in two steps: in a first step, the

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[0096] 10 November 2025 cellulose sample was digested at 30 °C with 72 wt.% sulfuric acid for 2 h, and in a second step, the cellulose sample was further digested at 120 °C with 1.8 wt.% sulfuric acid for 1 h. The remaining insoluble residue was considered the Klason lignin. Further experimental details regarding sample preparation and calculation can be found on page 931 of M. T. Amiri et al., Nat. Protoc. 2019, 14, 921-954 (DOI: 10.1038 / s41596-018-0121-7).

[0097] Determination of lignin removal rate

[0098] The Klason lignin of the untreated wood was determined in the same manner as the lignin content in cellulose. The weight difference between the weight of the residual Klason lignin in the cellulose sample and the weight of the original wood is considered the removed lignin. Acid-soluble lignin was not accounted for as it constituted a significantly smaller fraction than the Klason (acid-insoluble) lignin.

[0099] Determination of isolated lignin monomer yield

[0100] The lignin monomer yield was determined via hydrogenolysis. The isolated lignin sample and a catalyst (5 wt.% Ru / C) were mixed in 20 mL of 1,4-dioxane in a 50 mL autoclave reactor. The reactor was pressurized with 40 bar of H2, optionally with 0.036 vol% HC1 (as specified in examples), and the mixture was reacted at 250 °C for 3 h. The p-0-4 linkages in lignin were cleaved under such conditions, which produced lignin monomers. The yield of each type of monomer was quantified using GC-F1D with decane as the internal standard. Further experimental details regarding sample preparation and calculation can be found on page 948-949 of M. T. Amiri et al., Nat. Protoc. 2019, 14, 921-954 (DOI: 10.1038 / s41596-018-0121-7).

[0101] Determination of hydroxyl group content of lignin

[0102] The hydroxyl group content of the lignin was determined via quantitative31P-NMR following a procedure by X. Meng et al., Nat. Protoc. 2019, 14, 2627-2647 (DOI: 10.1038 / s41596-019-0191-l).

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[0104] 10 November 2025 Weight average molecular weight of lignin

[0105] The weight average molecular weight of lignin was determined using Gel permeation chromatography (GPC) with polystyrene standards of known monodispersed molecular weight.

[0106] Polydispersity index of lignin

[0107] The polydispersity index of lignin was determined according to as the ratio of the weight-average molecular weight to the number-average molecular weight, both determined using Gel permeation chromatography (GPC).

[0108] Residual lignin content of the cellulose

[0109] The residual lignin content of the cellulose was determined according to acid hydrolysis of the cellulose sample. Specifically, ca. 1.2 g of solid samples were pulverized in a ball mill (Restch) with a 50 mL ZrO2-lined grinding jar and 20 ZrO2 grinding balls of 1 cm in diameter (ca. 63 g in total). Milling was performed at 450 rpm for 1 h with 5 min on-off cycles. The powder was stored at 45°C and 45 mbar overnight to remove moisture. Approximately 0.3 g of the dehydrated sample was dispersed in 4.5 mL of 72 wt% H2SO4 and digested below 35°C for 1 h in a sonicating water bath. The mixture was then diluted with MQ water to 180 mL to a final acid concentration of 4wt% and digested again at 120°C for 1 h. The insoluble precipitate was filtered out and designated as Klason lignin.

[0110] NMR spectroscopy

[0111] NMR spectra were acquired using a Bruker Avance 111 400 MHz (11B) or Bruker A vance 500 MHz H,13C, HSQC) spectrometer using the standard pulse sequences from Bruker.

[0112] 31P NMR spectra were recorded using a Bruker Avance 111-HD 600 MHz spectrometer with 5 mm BBO cryoprobe. The experimental parameters used for the spectra

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[0114] 10 November 2025 acquisition were: pulse program=inverse gated decoupling pulse (zgig), SW=395 ppm, 01P=140 ppm, AQ=0.34 s, Dl=20 s, NS=64.

[0115] GC-F1D

[0116] Quantitative analysis of lignin monomer yield was performed with an Agilent 7890B series GC equipped with an HP5 column and a FID. N-Decane (0.2 mL) was added to the reaction mixture after hydrogenolysis as the internal standard. 1 mL of this solution was filtered into a vial with a 0.2 pm PTFE syringe filter and then injected into GC-F1D for quantification. The GC-F1D method was performed as follows: The injection temperature was 300 °C. 1 pL of the sample was injected with an autosampler in split mode (split ratio: 25:1). The column was initially kept at 40 °C for 3 min, then was heated at a rate of 30oC-mim1to 100 °C, followed by a heating rate of 40 °C-mim1to 300 °C that was held for 5 min. The monomer yields were calculated based on the respective effective carbon number (ECN) in reference to the n-decane standard (ECN=10). The ECNs of lignin monomers are summarized in the work of Shuai et al. (L. Shuai et al., Science, 2016, 354, 329-333. DOI: 10.1126 / science.aaf7810).

[0117] Lignin obtained using boric acid or alkali salts of boric acid is hereinafter also referred to as BAF lignin or BA-lignin. Pulp obtained using boric acid or alkali salts of boric acid is hereinafter also referred to as BAF pulp. measurement

[0118] The isolated lignin (ca. 0.2 g), either BAF lignin, soda control lignin, or Kraft lignin, was mixed with 0.5 mL of 1 M NaOH solution. As the boric protection group was removed by acidification, 0.012 g of boric acid was added to the BAF lignin mixture to achieve the same environment as in the BAF liquor during fractionation. All mixtures were sonicated at room temperature and centrifuged to precipitate the undissolved lignin fraction. 0.1 mL of the supernatant was reprecipitated in pH 3 water acidified with HC1

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[0120] 10 November 2025 over a dried and tared centrifugal filter. The mass of transferred supernatant was also recorded. The lignin precipitate was separated from water in a centrifuge and dried in a vacuum oven overnight. All experiments were conducted in duplicates to estimate the error margin.

[0121] Kappa number titration

[0122] The Kappa number of pulp samples was determined using manual titration, following the international standard ISO 302.11 Briefly, a dispersed pulp suspension with a known mass of dry pulp was oxidized with potassium permanganate at 25 °C for 10 min, during which time lignin and other oxidizable compounds would reduce potassium permanganate to Mn02. The reaction was terminated by adding a known amount of KI solution, and the iodide ion would be oxidized by the unreacted potassium permanganate to form elemental iodine. The formed iodine was then quantified using sodium thiosulfate titration. The consumption of sodium thiosulfate ultimately correlated with the quantity of reducing compounds, including lignin, in the pulp suspension. Numerically, the kappa number corresponds to the volume of 0.1 mol / L potassium permanganate (in m ) consumed by the oxidizable compounds in 1 gram of dry pulp.

[0123] Cellulose pulp freeness

[0124] The Schopper-Riegler degree of freeness test was conducted using an SR freeness tester (Frank-PTl), compliant with ISO 5267-1.12 1 L of a disintegrated pulp suspension of 0.2% consistency was added to the closed filling chamber. The sealing cone lifted to allow the suspension to drain through the screen, leaving a fibre pad, and the filtrate drained into the separating chamber. The water volume fraction drained through the side discharge pipe was weighed. The °SR corresponds to the volume of the water exiting the separating chamber through the bottom capillary discharge. A lower °SR means faster water drainage. Note that the Canadian Standard Freeness is another standard way to measure water drainage rate. Although it was not

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[0126] 10 November 2025 used in this study, it is worth mentioning to avoid confusion, since it has an inverted scale, where a higher value means faster drainage.

[0127] The mean pulp fibre length and width were measured using a Fibre Quality Analyzer (OpTest Equipment), fully compliant with TAPP1 T 271 ( echnical Association of the Pulp and Paper Industry (TAPP1). Test Method TAPP1 / ANS1 T 271: Fiber length of pulp and paper by automated optical analyzer using polarized light. (2012)). Fibres were continuously fed into a flow cell and imaged under a polarized light source. The average dimensions were automatically processed and reported by integrated image analysis. The aspect ratio corresponds to the ratio of the average length to the average width. measurement

[0128] The handsheet brightness was measured with a spectrophotometer (CM-2500d, Minolta), according to ISO 2470-1.14 This ISO brightness corresponds to the reflectance of blue light at a wavelength of 457 nm, using a D65 light source (daylight).

[0129] Handsheet thickness measurement

[0130] The thickness of the handsheet was measured using an automated micro calliper (L&W). Eight sampling locations were evenly selected on each handsheet and the average and the standard deviation were used in reporting.

[0131] Tensile index measurement

[0132] The tensile index was measured using a L&W Tensile Strength Tester based on Tappi T 494 .15 Handsheets were left in the climate-controlled room (50% RH and 23 °C) to equilibrate overnight. They were cut into testing strips 15 mm wide with a minimum of 100 mm in length and loaded onto the tester. The tensile strength was measured automatically. Four replicates were measured for each pulp type. The tensile index and the breaking length were calculated based on the measured grammage and

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[0134] 10 November 2025 thickness using the following equations. Breaking length corresponds to the maximum length of a paper sheet before it breaks under its own weight when suspended vertically. tensile strength F

[0135] Tensile index = -1max, tensile grammage strip width x grammage

[0136] F

[0137] Breaking length =1max, tensile strip width x grammage x gravitational acceleration

[0138] Optical microscopy

[0139] The surface of the handsheet was examined with an optical microscope (Stemi SV 11, Zeiss) with additional lateral lighting using LQ LED M illumination (Fiberoptic-Heim).

[0140] Examples 1-6: One-step lignin extraction with phenylboronic acid

[0141] Birch wood Betula pendula) (3 g, 125-450 pm) and phenylboronic acid (2.4 g) were mixed with 60 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 130 °C for 2 h (Example 2). The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm polyethersulfone (PES) membrane filter. The converted biomass solid was then washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both converted biomass solid and lignin were dried in a vacuum oven at 45 °C and 150 mbar for 24 h to yield 0.48 g of lignin and 1.86 g of cellulose. Cellulose acid hydrolysis showed 36.5 % removal of lignin based on Klason lignin. The residual lignin corresponded to 18.3 wt.% of the collected cellulose. Upon hydrogenolysis, the resulting lignin showed a yield of 46.1 wt.% of lignin monomers, which was equivalent to a residue of 67.9 % of equivalent p-0-4 linkages compared to ca. 70 % of the native P-0-4 linkages in the untreated birch wood used in this experiment.

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[0143] 10 November 2025 In a comparative experiment (Example 6), the same experiment was repeated without phenylboronic acid, yielding 0.51 g of lignin and 1.72 g of cellulose after drying.

[0144] 37.9 % of Klason lignin was removed from the biomass. The residual lignin corresponded to 19.4 wt.% of the collected cellulose. Upon hydrogenolysis, the resulting lignin showed a total yield of 16.2 wt.% of lignin monomers, which was equivalent to a residue of 40.2 % of equivalent p-0-4 linkages compared to ca. 70 % of the native p-0-4 linkages in the untreated birch wood used in this experiment.

[0145] Further experiments were conducted for different durations at 130 °C under otherwise identical conditions (Examples 1-5), the results of which are summarized in Table 1.

[0146] The noticeable differences in the lignin monomer yields and residual p-0-4 linkages after extraction underline the efficacy of the protective agent during the pulping process. Comparative Example 6 shows a significantly lower isolated lignin monomer yield (16.2 % vs. 46.1 %) and equivalent p-0-4 content (40.2 % vs. 67.9 %) compared with Example 2, which was conducted under the same conditions but in the presence of phenylboronic acid.

[0147] Table 1: One-step lignin extraction at different durations.

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[0149] 10 November 2025 Explanations to Table 1:a- comparative example without addition of phenylboronic acid.

[0150] Example 7: Sequential lignin extraction at low biomass loading with phenylboronic acid

[0151] Birch wood Betula pendula) (3 g, 125-450 pm) and phenylboronic acid (2.4 g) were mixed with 60 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 130 °C for 2 h. The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm PES membrane filter. The converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. The same conversion step was repeated on the recovered converted biomass solid at 150 °C for 0.75 h followed by 170 °C for 0.5 h and 180 °C for 0.5 h. The final converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both converted biomass solid and lignin were dried in a vacuum oven at 45 °C and 150 mbar for 24 h. The results from each cycle are summarized in Table 2. The total lignin monomer yield based on the loaded biomass was 4.8 wt.%.

[0152] Table 2: Example 7; sequential lignin extraction at low biomass loading.

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[0154] 10 November 2025 Example 8: Sequential lignin extraction at high biomass loading

[0155] Birch wood Betula pendula) (12 g, 450-1000 pm) and phenylboronic acid (2.6 g) were mixed with 60 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 130 °C for 3 h. The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm PES membrane filter. The converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. The same conversion step was repeated on the recovered converted biomass solid at 150 °C for 1.5 h followed by 170 °C for 0.25 h and 180 °C for 0.5 h. The final converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both converted biomass solid and lignin were dried in a vacuum oven at 45 °C and 150 mbar for 24 h. The results from each cycle are summarized in Table 3. The total lignin monomer yield based on the loaded biomass was 4.4 wt.%.

[0156] Table 3: Example 8; sequential lignin extraction at high biomass loading.

[0157] Examples 9-11: Various biomass species in sequential extraction using phenylboronic acid

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[0159] 10 November 2025 The conditions of Example 8 were applied to beech Fag us sylvatica} and black pine (Pinus nigra) at 3 g biomass loading and 125-450 pm particle size. The results of total lignin monomer yield combining all fractions relative to the respective dry biomass loading are summarized in Table 4. As a comparison for the results achieved using the method according to the invention (“Method”), the monomer yields achieved through reductive catalytic fractionation are also listed (“RCF”). RCF monomer yields can be considered as the maximum possible lignin monomer yield from native lignin.

[0160] Table 4: Various biomass species in sequential extraction.

[0161] As elucidated in the given examples, the method according to the invention provides excellent lignin removal rates, equivalent p-0-4 content, and lignin monomer yields, thus circumventing the disadvantages inherent in the established methods of the prior art. Moreover, the conditions of the method according to the invention are compatible with industrial pulping processes.

[0162] Examples 12 to 15: Various biomass species in improved sequential extraction

[0163] Biomass (10 g) and boric acid (1.2 g) were mixed with 60 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 80 °C for 2 h and at 130 °C for 1.5 h. The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm PES membrane filter. The converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. The same conversion step was repeated on the recovered converted biomass solid at 150 °C for 1.5 h followed by 170 °C for 0.25 h and 180 °C for 0.5 h. The final

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[0165] 10 November 2025 converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both converted biomass solid and lignin were dried in a vacuum oven at 45 °C and 150 mbar for 24 h. The results are summarized in Table 5.

[0166] Table 5: Lignin extraction using different wood species.

[0167] Examples 16-20: One-step lignin extraction with boric acid

[0168] Birch wood Betula pendula) (3 g, 125-450 pm) and boric acid (1.2 g) were mixed with 60 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 130 °C for 1.5 h (Example 17). The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm polyethersulfone (PES) membrane filter. The converted biomass solid was then washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both converted biomass solid and lignin were dried in a vacuum oven at 45 °C and 150 mbar for 24 h to yield 2.00 g of cellulose. Upon hydrogenolysis, the resulting lignin showed a yield of 3.16 wt.% of lignin monomers based on the dried wood mass.

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[0170] 10 November 2025 Further experiments were conducted for different durations at 130 °C under otherwise identical conditions (Examples 16-20), the results of which are summarized in Table 6. Example 20 was conducted in the same conditions as Example 19 without the addition of boric acid. The halved lignin monomer yield shows the efficacy of boric acid protection in the conversion step.

[0171] Table 6: One-step lignin extraction at different durations.

[0172] Explanations to Table 6:a- comparative example without addition of boric acid.

[0173] Example 21: Sequential lignin extraction at low biomass loading with boric acid Birch wood Betula pendula) (3 g, 125-450 pm) and boric acid (1.2 g) were mixed with 60 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 130 °C for 3 h. The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm PES membrane filter. The converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. The same conversion step was repeated on the recovered converted biomass solid at 150 °C for 1.5 h followed by 170 °C for 0.25 h and 180 °C for 0.5 h. The final converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both

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[0175] 10 November 2025 converted biomass solid and lignin were dried in a vacuum oven at 45 °C and

[0176] 150 mbar for 24 h. The results from each cycle are summarized in Table 7. The total lignin monomer yield based on the loaded biomass was 4.1 wt.%. Table 7: Example 21; sequential lignin extraction at low biomass loading with boric acid.

[0177] Table 8: Solubility of the extracted lignins

[0178] Explanations to Table 8:a- Lignin extracted at 130 °C;b- combined Lignin extracted at 150, 170 and 180 °C.

[0179] Examples 22-24: Various configurations for sequential lignin extraction and coextraction of xylan

[0180] The three fractionation configurations (cross current, mixed current, counter current) were simulated by successively recycling the solid and liquor fractions and recombining them in designated sequences.

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[0182] 10 November 2025 Birch wood Betula pendula) (10 g, <600 pm) was used in a series of four extraction steps as outlined in Fig la for the crossed current configuration (Example 22), in Fig lb for the mixed current configuration (Example 23) and 1c for the counter current configuration (Example 24). In each step they were mixed in a 100 mL stainless-steel reactor with either a fresh liquor consisting of a protective agent (20 mmol) and 50 mL of IM NaOH aqueous solution or a spent liquor from a previous extraction step. In every step SI untreated wood chips were (UWC) used and the mixture was heated without stirring at 80 °C for 2 h and at 130 °C for 1.5 h. In every step S2 the mixture was heated without stirring at 150 °C for 1.5 h. In every step S3 the mixture was heated without stirring at 170 °C for 0.25 h. In every step S4 the mixture was heated without stirring at 180 °C for 0.5 h. For each step the resulting converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.45 pm polyethersulfone (PES) membrane filter. Each fractionated solid (FS) was used for each next step without washing as depicted in Fig la to 1c. Each fractionated liquor (LS) was used for each next step as depicted in Fig la to 1c. The final solid fractions (P1 / P2) were each washed with MQ water until the pH was below 8. The solid cellulose fraction was dried at 45°C and 50 mbar.

[0183] The product liquors (LI to L5) were acidified to pH 4 using HC1 (37 wt.%). The resulting precipitate was separated using a 0.45 pm PES membrane filter. Without further drying or neutralizing HC1, the precipitate was dispersed in 1,4-dioxane (4mL) three times to dissolve lignin and remove insoluble xylan. The solid xylan and the lignin solution were eventually separated using a 0.8 pm nylon membrane filter. The solid xylan containing fraction was dried at 45°C and 50 mbar. The liquid lignin containing fraction was concentrated and a small amount of NaHCOs (<0.01 g) was added to the mixture when lignin started to precipitate during evaporation while water (originally mixed with the co-precipitate) still remained. Water was then fully evaporated. The lignin sample was further dried in vacuum oven at 50°C, 60 mbar overnight.

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[0185] 10 November 2025 Referring to Fig la, the mass balance for the crossed current configuration was developed based on the untreated wood chips and fresh liquor streams as the incoming feed, and Pl + LI to L5 as the existing streams.

[0186] Referring to Fig lb, the mass balance for the mixed current configuration was developed based on the untreated wood chips and fresh liquor streams as the incoming feed, and Pl + LI and L4, or P2 + LI and L5 as the existing streams (duplicates). An average was taken from the two sets of duplicated outgoing streams.

[0187] Referring to Fig 1c, the mass balance for the counter current configuration was developed based on untreated wood chips and fresh liquor streams as the incoming feed, and Pl + L4, or P2 + L5 as the existing streams (duplicates). An average was taken from the two sets of duplicated outgoing streams.

[0188] Table 9: Various configurations for sequential lignin extraction.

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[0190] 10 November 2025

[0191]

[0192] Explanations to Table 10: “KL” refers to Klason lignin; “Glu” refers to glucan; "Xyl" refers to xylan; “Gal” refers to galactan; “Ara” refers to arabinan; and “Man” refers to mannan.

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[0194] 10 November 2025 Table 11: Example 24: Mass balance of biomass components in the counter-current configuration.

[0195] Explanations to Table 11: “KL” refers to Klason lignin; “Glu” refers to glucan; "Xyl" refers to xylan; “Gal” refers to galactan; “Ara” refers to arabinan; and “Man” refers to mannan.

[0196] Table 12: Example 23: Mass balance of boric acid (BA) in the mixed-current configuration.

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[0198] 10 November 2025 Table 13: Example 24: Mass balance of boric acid in the counter-current configuration.

[0199] Examples 25 to 30: One-step lignin extraction with different boron-based protection agents Birch wood Betula pendula, <600 pm) (10 g) and a protective agent (20 mmol) were mixed with 50 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 80 °C for 2 h and at 130 °C for 1.5 h. The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm polyethersulfone (PES) membrane filter. The converted biomass solid was then washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both converted biomass solid and lignin were dried in a vacuum oven at 45 °C and 150 mbar for 24 h. A control experiment was performed without the addition of the boron-based protective agent, while all other steps were kept the same. The results are summarized in Table 14.

[0200] Table 14: Lignin extraction using different protective agents.

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[0202] 10 November 2025

[0203] Examples 31 to 34: Screening of additional wood species

[0204] Biomass (10 g) and boric acid (1.2 g) were mixed with 60 mL of IM NaOH aqueous solution in a 100 mL stainless-steel reactor. The aqueous mixture was heated with stirring at 80 °C for 2 h and at 130 °C for 1.5 h. The converted biomass solid was separated from the aqueous mixture by centrifugation followed by filtration with a 0.2 pm PES membrane filter. The converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. The same conversion step was repeated on the recovered converted biomass solid at 150 °C for 1.5 h followed by 170 °C for 0.25 h and 180 °C for 0.5 h. The final converted biomass solid was washed with deionized water until neutral. The washing water was combined with the separated aqueous mixture. Residual lignin was precipitated from the aqueous mixture by acidification to pH 2 using hydrochloric acid, followed by centrifugation and filtration with a 0.2 pm PES membrane filter. Both converted biomass solid and lignin were dried in a vacuum oven at 45 °C and 150 mbar for 24 h. The results are summarized in Table 15.

[0205] Table 15: Lignin extraction using different wood species

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[0207] 10 November 2025 Examples 36-37: Cellulose characterization

[0208] Wood chips (ca. 0.5 cm x 0.5 cm x 3 cm) and boric acid were mixed with 700 mL of 1 M NaOH aqueous solution in a 1 L stainless-steel reactor. The mixture was treated in four extraction cycles following the procedure described for Example 22 with the changes, that the mixture is stirred at ca. 60 rpm and that a cellulose filter was used instead of membrane filters. The pulp fibers were washed with water until pH<8 and the water was drained through the filtration system using vacuum.

[0209] The resulting pulp cake was mixed with H2O2 (5 % in NaOH aqueous solution, pH=12) to achieve a final consistency of 10%. The mixture was placed in zipper-sealed polypropylene (PP) bags and heated in a thermostatic water bath at 70 °C for 1 h. The bleaching solution was removed with vacuum filtration using a cellulose filter on a Buchner funnel. This process was repeated once. The bleached pulp cake was finally diluted with 0.1 M NaOH solution to 10% consistency and heated at 70 °C for 1 h inside a zipper-sealed PP bag in a water bath. The base wash was removed by filtration, and the bleached pulp was washed with water until a pH<7.2 was reached with the filtrate. The pulp cake was oven-dried at 105 °C to measure the pulp yield.

[0210] For example 36 Birch Chips Betula pendula) (120 g) and boric acid (14.64 g) were used.

[0211] For example 37 Spruce chips (76 g) and boric acid (10 g) were used.

[0212] Table 16: Characteristics of BAF pulp compared with industrial Kraft pulp.

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[0214] 10 November 2025

[0215] All dried pulp samples were hand torn into small pieces and rehydrated by soaking in MQ water for 5 h. The mixture was disintegrated at a 1% consistency for 60 000 revolutions using a pulp disintegrator ( rank-PTl). The suspension was diluted to 0.3% with water for storage. The pulp handsheets were made using an automated sheet former of a 20 cm nominal diameter ( rank-PTl). The pulp suspension was evenly sampled and massed with a targeted dry mass of 1.9 g. The mass was selected to achieve a grammage of 60 g / m2, which corresponds to the ISO and TAPP1 standard for handsheet mechanical tests. The pulp suspension was added to tap water to reach a total volume of 7 L. The water was drained through a fine screen, above which fibres were retained. The pulp pad on the screen was transferred between two pieces of blotting paper and dried at 98 °C, 0.1 bar for 9 min. The dry mass of each handsheet was measured, and the corresponding grammage was calculated. Table 17: Characteristics of handsheets made of BAF pulp and industrial Kraft pulp.

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[0218] Examples 38: p-0-4 content

[0219] The11B NMR of boric acid in NaOH solution, never acidified BA-lignin, and once or twice acid-precipitated BA-lignin were compared.11B peak assignment was based on literature reports ((Van Duin, M., Peters,). A., Kieboom, A. P. G. &Van Bekkum, H. Studies on borate esters 1. Tetrahedron 40, 2901-2911 (1984) and Hiraishi, N., Sayed, M., Hill, R., Tagami, J. & Hayashi, F. Interactions of boron released from surface pre-reacted glass ionomer with enamel / dentin and its effect on pH. Sci. Rep. 11, 1-9 (2021). The significantly different11B chemical shifts before and after lignin acidification resulted from the reversible hydroxide ion coordination with boron under different pH. The never acidified lignin was prepared with direct antisolvent precipitation using acetone (Figure 2b). This lignin-xylan precipitate was then dissolved using D2O and reprecipitated in acetone-d6 twice to remove the effect of 'H on NMR spectra. Without neutralization, lignin precipitated as the sodium lignin borate ester salt with one major NMR peak at 1.7 ppm corresponding to cyclic borate ester, which is also in the same range as the boric ester of 2,4-pentenediol as a model compound reaction (Figure 2c). Although the exact chemical shift for cyclic borate ester of lignin is not readily available, Van Duin conducted a systematic study of the11B chemical shifts of various cyclic boric and borate esters and reported values in same range, slightly higher than the chemical shift of sodium borate. The BA-lignin precipitated with HC1 acidification to pH 3 showed a small peak at 19.9 ppm (Figure 2d), corresponding to the cyclic lignin-boric ester (with a tri-coordinated boron). After redissolving the acid precipitate of lignin with NaOH solution (pH 13) and reprecipitated with HC1 to pH, no boron signals could be detected in the lignin sample by11B NMR (Figure 2e), suggesting a near complete

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[0221] 10 November 2025 hydrolysis of boric ester on lignin. The measurements were recorded using the same sequence in the same instrument consecutively, which should have comparable signal- to-noise ratio.

[0222] The BA-protected and unprotected p-0-4 linkages in the three types of lignin samples were further compared through the characteristic peak of the hydrogen on the benzylic carbon in1H NMR (Figure 2f-h). No unprotected p-0-4 linkages were detected in antisolvent precipitated lignin (Figure 2f). As a result of the OH' coordination to boron involved in the never acidified lignin, the chemical shift of the BA-protected p-0-4 significantly differed from the lignin without OH' coordination, overlapping with sugar peaks. In the once-acidified lignin sample, a large peak corresponding to the unprotected p-0-4 linkages appeared around 4.85 ppm with a small peak at 5.15 ppm for the BA-protected p-0-4 linkages (Figure 2g). No BA- protected p-0-4 peaks was detected in twice- acidified lignin, leaving only one large peak for the unprotected p-0-4 linkages (Figure 2h). The p-0-4 content in these lignin samples were thereby quantified using quantitative NMR (Table 18). The BA- protected p-0-4 linkages in the never acidified lignin was quantified using quantitative11B NMR following the literature method (Aguilera-Saez, L. M. et al. Pushing the frontiers: Boron-11 NMR as a method for quantitative boron analysis and its application to determine boric acid in commercial biocides. Analyst 143, 4707- 4714 (2018) while all others were measured with1H NMR. The total p-0-4 content remained around 1.5 mmol / g over the acidification sequence. Remarkably, a sharp drop of 86% in the BA-protected p-0-4 content can be seen between the never- acidified and once-acidified lignin samples. The lignin hydroxyl content was quantified with31P NMR. Notedly, the aliphatic OH content was measured to be about ~twice the P-0-4 content in both samples since each p-0-4 linkage has two hydroxyl groups, further confirming the quantification results.

[0223] Table 18: p-0-4 content in lignin samples before and after acid wash.

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[0226] Explanations to Table 8:a- Measured by quantitative11B NMR with 2,4,6- triphenylboroxin as the internal standard;b- Measured by quantitativebH NMR with l,2,4,5-tetrachloro-3-nitrobenzene as the internal standard;c- Measured by quantitative1H-13C HSQCo with polystyrene of the internal standard, retrieved from Bourmaud et al. (Bourmaud, C. L. et al. Quantification of Native Lignin Structural Features with Gel-Phase 2D-HSQC 0 Reveals Lignin Structural Changes During Extraction. Angew. Chemie Int. Ed. 63, (2024).);d- Measured by quantitative31P NMR as per the protocol of Meng et al. (Meng, X. et al. Determination of hydroxyl groups in biorefinery resources via quantitative 3 IP NMR spectroscopy. Nat. Protoc. 14, 2627- 2647 (2019).

[0227] The content of p-0-4 linkages is calculated as the ratio of unprotected p-0-4 linkages in the sample to the total aromatic units in the sample, expressed as a percentage. For comparison with native lignin, this value is then divided by the typical p-0-4 content found in native lignin of the same wood type (e.g., 80% for hardwood, 60% for softwood) and multiplied by 100 to give the percentage of p-0-4 linkages retained relative to native lignin. For example, the content of p-0-4 linkages of the never acidified sample is: 1.64 / 2.53*0.8 = 52%

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Claims

November 10, 2025C l a i m s1. Method for alkaline pulping of biomass in an aqueous mixture, comprising a conversion step in which biomass comprising lignin is converted in an aqueous liquor to obtain converted biomass, wherein the aqueous liquor comprises i) a protective agent ii) a basic compound hi) water iv) optional additional pulping reagents.

2. Method according to claim 1, characterized in that the conversion step is conducted at a temperature of 80 °C to 250 °C, preferably of 80 °C to 200 °C, more preferably 80 °C to 180 °C or at a temperature of 80 °C to 250 °C, preferably of 100 °C to 200 °C, more preferably of 130 °C to 180 °C.

3. Method according to any one of the preceding claims, characterized in that the pH of the aqueous mixture is 10 or higher, preferably 12 or higher, more preferably 13 or higher, even more preferably 13.5 or higher.

4. Method according to any one of the preceding claims, characterized in that the protective agent reacts with lignin to form a hydrolysable intermediate, in particular an ester intermediate, particularly preferably a boronic ester intermediate or a borate ester intermediate.

5. Method according to any one of the preceding claims, characterized in that the protective agent comprises or consists of a compound comprising a group 13 element, in particular boron, preferably the protective agent comprises or consists of a boric acid compound, in particular a boric acid compound selectedfrom the group consisting of boric acid, alkali salts of boric acid, or mixtures thereof, and / or a boronic acid compound, in particular a boronic acid compound selected from the group consisting of optionally substituted phenylboronic acid, optionally substituted Ci to Ce-alkyl boronic acids, optionally substituted heterocyclic boronic acids, alkali salts of a boronic acid, and mixtures thereof.

6. Method according to any one of the preceding claims, characterized in that the basic compound is soluble in water and / or is selected from the group consisting of alkali metal oxides, alkaline earth metal oxides, alkali metal hydroxides, alkaline earth metal hydroxides, ammonia, and mixtures thereof, and / or is present in the aqueous liquor in a concentration of 0.01 to 10 M, preferably 0.1 to 5 M, more preferably 0.5 to 2 M.

7. Method according to any one of the preceding claims, characterized in that the aqueous liquor is substantially free of organic solvents.

8. Method according to any one of the preceding claims, characterized in that the optional additional pulping reagents are selected from the group consisting of hydrogen sulfide, alkaline sulfide, alkaline bisulfide, sulfurous acid, alkaline sulfite, alkaline bisulfite, alkaline earth sulfite, alkaline earth bisulfite, ammonium sulfite, ammonium bisulfite, alkaline carbonate, alkaline bicarbonate, and mixtures thereof.

9. Method according to any one of the preceding claims, characterized in that the method comprises a separating step in which the converted biomass is separated from the aqueous mixture.

10. Method according to claim 9, characterized in that the method is performed in batch mode and / or that the conversion step is repeated by combining the converted biomass with aqueous liquor for conversion at the same or a different temperature for the same or a different duration as the unconverted biomass.HT / rp 241043WO10 November 202511. Method according to any one of claims 1 to 9, characterized in that the method is performed in a continuous mode, optionally using a counter-current reactor, cross-current reactor or mixed-current reactor.

12. Method according to any one of the preceding claims, characterized in that lignin is extracted from the aqueous mixture in a lignin extraction step after the conversion step, more preferably after the separating step.

13. Method according to claim 12, characterized in that the lignin extraction step comprises precipitation of lignin from the aqueous mixture, in particular by precipitation through acidification preferably by acidification with HC1 and / or CO2, most preferably by acidification with CO2 or precipitation through use of an antisolvent or by a combination thereof, particularly preferably by precipitation through use of an antisolvent.

14. Method according to any one of the preceding claims, characterized in that lignin and xylan are co-extracted from the aqueous mixture in a lignin-xylan coextraction step after the conversion step, more preferably after the separating step.

15. Method according to claim 14, characterized in that the lignin-xylan coextraction step comprises precipitation of lignin and xylan from the aqueous mixture, in particular by precipitation through acidification or precipitation through use of an antisolvent or by a combination thereof, particularly preferably by precipitation through acidification, more preferably by acidification with HC1 and / or CO2, most preferably by acidification with CO2.

16. Method according to any one of the preceding claims, characterized in that the hydrolysable intermediate is undergoing hydrolysis preferably by acidification, more preferably by acidification with HC1 and / or CO2, most preferably byHT / rp 241043WO10 November 2025acidification with CO2, and / or that the protective agent is regenerated during hydrolysis and preferably reisolated and / or recycled, more preferably recycled.

17. Lignin fragments comprising repeating units of the general formula 1, and / or repeating units of the general formula 11whereinR1is selected from the group consisting of hydroxy, methyl, ethyl, propyl, butyl, phenyl, p-tolyl, p-fluorophenyl, p-chlorophenyl, p-aminophenyl, thiophen-2-yl, 2- pyridyl, cyclohexyl, and benzyl and the methoxy groups shown in parentheses may be present or absent, and repeating unit 1 can be present in a neutral or anionic form, wherein the lignin fragment has a content of p-0-4 linkages of 20% to 75% relative to that of native lignin.

18. Lignin fragments according to claim 17, comprising repeating units of the general formula 1, wherein R is hydroxyl or phenyl.

19. Lignin obtainable by the method according to any one of claims 1 to 16, characterized in that the lignin has a total hydroxyl group content of 1 mmol / g to 10 mmol / g, preferably of 1.5 mmol / g to 5 mmol / g, and / or a weight-average molecular weight of 1000 g / mol to 30000 g / mol, preferably of 2000 g / mol to 20000 g / mol and a content of p-0-4 linkages of at least 20%, preferably 25% toHT / rp 241043WO10 November 202575% relative to that of native lignin, and / or a polydispersity index of 2 to 40, preferably of 2 to 20.

20. Cellulose obtainable by the method according to any one of claims 1 to 16, characterized in that the cellulose has a residual lignin content of 0.5 wt.% to 20 wt.%, preferably 1 wt% to 20 wt%, more preferably of 1 wt% to 10 wt%, even more preferably 3 wt.% to 10 wt.%, and / or a mean fiber length of at least 1.1 mm.

21. Xylan obtainable by the method according to any one of claims 1 to 16, characterized in that the xylan has a purity of more than 50 %, preferably than 80 %, more preferably more than 90 %, even more preferably more than 95 %.HT / rp 241043WO10 November 2025

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