Method for producing a cellulose sheet

Abstract

The invention relates to a method for producing a cellulose sheet having a xylan content of less than 25.0 percent by weight and having a tear index of at least 4.00 mN∙m2∙g-1, characterised by the combination of measures that straw, which contains lignin, xylan and cellulose and is suspended in an aqueous phase, is treated by means of an alkaline aqueous solution containing ethanol in order to reduce the lignin content, as a result of which a first cellulose is obtained, the first cellulose is treated with a xylanase in order to reduce the xylan content, as a result of which a second cellulose is obtained, and a cellulose sheet is formed from the second cellulose.

Description

[0001] Method for producing a cellulose sheet

[0002] The invention relates to a process for producing a cellulose sheet and a process for obtaining chemical raw materials from lignocellulose.

[0003] Background of the invention

[0004] Biorefineries fulfill the task of sustainably producing materials, chemicals, and bioenergy from biomass, utilizing as many raw material components as possible. The pulp and paper industry is an example of an existing biorefinery. Traditional refining processes, based on wood, the most abundant raw material, are primarily geared towards pulp production. They require harsh reaction conditions that allow only limited utilization of byproducts, which are mainly used for energy recovery (Singh et al., 2022; Chandel et al., 2018; Steffen et al., 2024).

[0005] Newer lignocellulose biorefinery concepts aim to obtain the lignocellulosic components, such as lignin, glucose (the main component of cellulose), and xylose (the main component of xylane, a type of hemicellulose), with minimal energy expenditure in the purest possible form, in order to produce valuable chemicals. The raw material used is primarily lignocellulose from annual plants, rather than wood. The main advantage is that the lignin in lignocellulose is less condensed than in wood, allowing the extraction of lignin, which is always required in the first step, to be carried out under significantly milder conditions than in conventional pulp production (Singh et al., 2022; Chandel et al., 2018; Steffen et al., 2024).

[0006] Changes in the pulp market, driven by increased demand for "green" packaging materials and the generally rising need for packaging paper, have led to a shortage of recycled paper, negatively impacting quality and price. This is prompting the pulp industry to seek new sources of virgin fiber. Pulp made from cereal straw, which is cheaper and more readily available, represents an alternative to fibers from recycled materials and wood pulp. In some Asian markets, where wood is less readily available, bagasse and rice straw already account for 70% of the raw materials used in the pulp industry (Steffen et al., 2024; Worku et al., 2023).

[0007] Especially in the booming packaging paper sector, it is expected that the use of fibers from straw can compensate for the reduced quality of recycled primary fibers. This, in turn, can also reduce the use of chemical additives (such as dry or wet strength additives). Various processes are used to produce pulp from straw, which differ from classic wood pulping methods (Kraft pulping / sulfate process, sulfite process) primarily in the use of milder conditions.

[0008] Organosolv pulping is considered to have the greatest potential for producing pulp from straw and other lignocellulosic waste from the food industry. This process involves fractionating the lignocellulosic raw material by treatment in an aqueous organic phase, with or without a catalyst (Thoresen et al., 2020).

[0009] The following are examples of organosolv processes that can also be applied to wheat straw.

[0010] In the Alcell process, an aqueous solution of ethanol (approx. 50 wt%) is used for delignification. The cooking process takes place at 190 °C and 28 bar. Since neither acids nor alkalis are added, the pH value stabilizes at 4 due to the deacetylation of the raw material. Lignin, furfural, acetic acid, and hemicellulose sugars (such as xylose) are the main byproducts of the Alcell process with wheat straw (Azadi et al., 2013).

[0011] Other organosolv processes are based on the use of organic acids such as acetic acid (Acetosolv, Acetocell, Formacell, CIMV) or formic acid (Formacell, Milox, Formico, CIMV) as solvents (Tofani et al., 2024).

[0012] The most established process for straw and other non-wood materials is pulping with sodium hydroxide (NaOH) as the active chemical. An organosolv process for the production of wood pulp with ethanol and NaOH at 170 °C was described as early as 1982 in WO 1982 / 001568 Al.

[0013] The high silicate content in, for example, wheat straw leads to problems in the treatment of black liquor and the recovery of NaOH due to the high solubility of silicate in alkaline environments. To avoid this problem, sodium carbonate can be used instead of sodium hydroxide. Pulp produced in this way has higher lignin content and is therefore less suitable for bleached papers, but rather for packaging material or in blends with recycled paper to improve mechanical properties (Steffen et al., 2024).

[0014] The so-called NACO process also uses sodium carbonate, but oxygen and a small amount of sodium hydroxide are added for activation during delignification. This results in pulps with a low lignin content (low kappa number). This process is carried out in two special reactors (so-called turbo pulpers) operated in series at a pressure of 6–7 bar and a temperature of 130–145 °C (depending on the raw material) (Fiala et al., 1983). The IDE process is also a sulfur-free alkaline pulping method. The process consists of three steps: impregnation, depolymerization, and extraction. In the first step, the raw material is treated with a concentrated sodium carbonate solution. The second step involves boiling in an aqueous ethanol solution for delignification. In the final step, the degraded lignin is extracted with a fresh aqueous ethanol solution.In experiments with wheat straw, yields of over 50% are achieved, with the residual lignin content being below 2% (Hultom et al., 1997).

[0015] The ASAM process (alkali sulfite anthraquinone methanol) or ASAE process (alkali sulfite anthraquinone ethanol) is carried out at pH values ​​above 13 and at 170–180 °C, with anthraquinone acting as a catalyst and sulfite as a delignifying agent (Black, 1991; EP 0538576 Al; Kirgi et al., 1994). In a study by Kirgi et al. (1994), ASAE pulps of Brutia pine (Pinus brutia Ten.) showed higher yields, viscosity, brightness, and improved strength properties compared to conventional sulfate pulps.

[0016] EP 2611820 Bl describes a process for separating lignin from lignocellulosic material by digestion with alcohol (ethanol or isopropanol), water, and a base (NaOH or KOH) at a temperature below 100 °C. The mild and selective conditions of this process allow for the decoupling of lignin degradation from the simultaneous hemicellulose degradation that otherwise occurs in conventional boiling processes. Depending on the conditions, up to 93% of the lignin could be separated from the wheat straw, with only a small proportion of xylan being degraded and removed from the straw under these conditions. The resulting low residual lignin content is a prerequisite for the efficient enzymatic production of xylose from xylan and glucose from cellulose, and for the potential production of bioalcohol.

[0017] US Patent 9970038 B2 describes the extraction of carbohydrate breakdown products such as xylose or glucose from wheat straw. First, ground wheat straw was delignified in an alkaline (NaOH) aqueous isopropanol solution or in the presence of hydrogen peroxide in an alkaline aqueous ethanol solution. The pretreated wheat straw was then treated with a commercial enzyme mixture consisting of cellulases and hemicellulases (Accellerase 1000 from Genencor). The xylose present in solution was enzymatically reduced to xylitol, a sugar substitute, using a xylose reductase from Candida tenuis. Due to the use of an enzyme mixture that also contains cellulases, the extraction of pulp using this method is not possible.

[0018] US Patents 9187571 B2 and 8617851 B2 propose a method to make cellulose more accessible to enzymatic or chemical modification. Unlike the methods described above, this method starts with cellulose, not lignocellulose, which is treated with a mixture of NaOH, water, and ethanol. Compared to untreated cellulose, this pretreated cellulose can be more easily depolymerized to glucose using cellulase.

[0019] The optimal solution for a pulp biorefinery would be a process that enables the production of pulp while simultaneously yielding native, sulfur-free lignin and xylan / xylose as a chemical feedstock. The simultaneous production of the three fractions (pulp, lignin, xylan / xylose) is not mentioned in the examples above.

[0020] Lignin is considered the chemical feedstock of the future, partly due to its abundance in nature—it is the second most abundant biogenic raw material after cellulose. Current applications include thermal utilization, its use as a binder and stabilizer in concrete and cement, and as a fertilizer. However, intensive research is underway on the use of lignin as a chemical feedstock, with applications such as adhesives, bioplastics, and as a basis for energy storage in battery cells being developed (Norgren & Edlund, 2014; Gaspar & Fardim, 2023).

[0021] The recovery of pure xylan or pure xylose in industrially applied pulping processes for wood pulp is very limited due to the high temperatures required. For example, larger quantities of xylose from the pre-hydrolysis liquor in the Kraft-based dissolving pulp process can practically only be recovered after complex purification procedures (Chen et al., 2018).

[0022] The lye obtained from the sulfite process contains not only xylose but also arabinose, glucose, galactose, and mannose, as well as degradation products such as acetic acid, furfural, and 5-hydroxymethylfurfural, which act as fermentation inhibitors. To enable the conversion of the sugars to L-lactic acid in a subsequent fermentation using Enterococcus mundtii, the fermentation inhibitors must be removed using a complex process called simulated moving bed chromatography (Hoheneder et al., 2021).

[0023] Often, the xylan removed and broken down during pulp production is burned as "waste" (Puls & Saake, 2010) and is therefore not available as a chemical raw material.

[0024] The hydrolytic cleavage of xylan from wheat straw has also been described. Walker et al. (2018) used steam explosion (pressure 12 bar(g), residence time 6 min) with phosphoric acid as a catalyst to release 90% of the xylose from wheat straw into the hydrolysate, which was subsequently reduced microbially to xylitol. A disadvantage is the formation of the fermentation inhibitors furfural and 5-hydroxymethylfurfural (HMF) due to the dehydration of the pentoses and hexoses, respectively.

[0025] Agrawal et al. (2015) used a continuous reactor to pretreat wheat straw with dilute sulfuric acid at 160 °C, resulting in the hydrolysis of a large proportion of the hemicellulose. Glucose was released from the pretreated straw using commercial cellulases, which were then fermented with Saccharomyces cerevisiae to produce ethanol. The authors demonstrated that lignin, acetic acid, furfural, HMF, and vanillin, which are released or formed during the dilute acid pretreatment, inhibit the cellulases used.

[0026] The additional extraction of xylan / xylose in connection with pulp production is ruled out based on the current understanding of its role in the pulp matrix. Xylan is considered a binding agent between the fibers in pulp, promoting paper strength. The prevailing theory states that the hydrogen bonds it mediates have positive effects on the strength and elasticity of cell walls in plants, thus leading to increased resistance to enzymatic degradation (Busse-Wicher et al., 2014).

[0027] The hemicelluloses (e.g., xylans) naturally occurring in pulp improve tensile strength and contribute to the formation of stronger fiber bonds. However, the pulping conditions used for pulp production lead to the removal or modification of these hemicelluloses (Puls & Saake, 2010; Schaubeder et al., 2024).

[0028] Wheat straw xylan, like softwood xylan, is arabinoxylan, with the two pentoses xylose and arabinose making up approximately 30% and 7% of the total dry matter in wheat straw, respectively (Walker et al., 2018).

[0029] In addition to the depolymerization of celluloses and hemicelluloses, chemical changes also occur during cooking and bleaching processes. For example, in the Kraft process, 4-O-methylglucuronic acid is converted to hexenuronic acid through the elimination of methanol, which can then subsequently react with bleaching agents (Henriksson et al., 2024; Schaubeder et al., 2024).

[0030] Malik et al. (2020) pretreated wheat straw using various methods: acidic (0.1 N H₂SO₄, 125 °C for 120 min), alkaline (6 wt% NaOH, 150 °C for 120 min), and hot water (150 °C for 120 min), extracting glucose, xylose, and arabinose. Pretreated and untreated wheat straw were pulped with different concentrations of NaOH (12%, 14%, and 16%) at 166 °C for 180 min. Paper sheets were subsequently produced from the different pulps and analyzed for their strength properties (tear index, tensile index, and burst index). Compared to untreated wheat straw, the tensile and burst index decreased across the board, with the greatest decrease in the acid-treated wheat straw, where almost the entire hemicellulose fraction was removed.In the case of the Tear Index, a slight increase (up to +4.3%) was observed for the pretreated wheat straw (for all three variants) at a NaOH concentration of 16%. A study by Sjöberg et al. (2004) shows that the strength properties of paper are positively influenced by the presence of hemicelluloses on the surface of the fibers; therefore, hemicelluloses such as xylans are used as additives in papermaking to improve mechanical properties.

[0031] Naterova et al. (1986) added maize xylans to wrapping paper, which increased the flexural strength by 172% with an addition of 2% xylan.

[0032] US 5810972 A describes the addition of highly ground xylan-rich birch pulp, xylan or galactomannan to cellulose for paper towels, which increased the tensile strength both in machine tensile strength by 84% and across the grain direction by up to 90%.

[0033] A study by Puls & Saake (2010) shows that the effect of arabinoxylan on paper strength depends on the refining grade and the pulp type. The higher the refining grade, the less pronounced the positive effect of arabinoxylan. The effect is greater with sulfite pulps than with sulfate pulps, and greater with softwood pulps than with hardwood pulps. When using unmodified xylans, also in combination with epichlorohydrin resin, a paper additive, the tensile index can be increased by 50–55% and the tear index by 20–25% in products with a low refining grade.

[0034] EP 1688534 Al described the addition of arabinoxylans to a pulp during papermaking. This improved paper properties such as tear length, tensile strength, bulk, and appearance. The study showed that adding an arabinoxylan derived from oat hulls resulted in greater tear lengths compared to adding a 4-O-methylglucuronoxylan from birch wood or a Lenzing xylan from beech wood pulp. The arabinoxylan from oat hulls does not contain the 4-O-methylglucuronic acids found in hardwood and softwood xylans as substituents and is characterized by a longer chain length than the Lenzing xylan.

[0035] The pulps mentioned in the aforementioned patents and publications for increasing strength through the addition of xylan are all bleached wood pulps produced using conventional, harsh cooking processes such as the Kraft process (170 °C). These processes involve intensive degradation and significant chemical modification of the xylan.

[0036] Xylanase-containing enzyme mixtures have been used to achieve various goals: for the modification of wood pulps to improve dewaterability (Blomstedt et al., 2010), for the extraction of chromophores (US 5498534 A) to improve milling (Noe et al., 2008), or for the purpose of enzyme-assisted bleaching of sugar cane and wheat straw pulps (Viikari et al., 1994; Moriya et al., 2007; Singh et al., 2011).

[0037] Most xylanases known in the literature also act as cellulases, which is undesirable in pulp production. However, xylanases such as those from Bacillus SSP-34 (Subramaniyan & Prema, 2000) or Thermomyces lanuginosus (Gomes et al., 1993) have also been described that exhibit no or only minimal cellulase activity.

[0038] The improved bleachability is attributed to the degradation of xylans, which precipitates on the fiber surface during the boiling of Kraft pulps, and the removal of lignin-blocking xylans (Viikari et al., 1993). No effects on fiber strength have been reported.

[0039] Furthermore, Viikari et al. (1993) conclude that high hemicellulose contents are advantageous for papermaking with regard to fiber-fiber bonding properties, and that removing hemicellulose does not improve these properties. Therefore, reducing xylan content using enzymes is ruled out a priori; instead, the addition of xylan to increase strength is recommended (see above).

[0040] The enzymatic removal of xylan from pulp and its effect on binding strength has only been investigated in a few studies, and these were conducted for purely scientific purposes. The pulps examined were bleached softwood kraft pulps, and the xylanase treatment was performed after milling.

[0041] Schönberg et al. (2001) investigated the role of xylan using spruce wood kraft pulp as an example. Treatment with xylanase degraded 0.9% of the xylan, resulting in a 23% decrease in the internal binding strength (Scottish Bond Value). Adsorption of 1.5% xylan led to a 22% increase in the Scottish Bond Value. Schaubeder et al. (2024) found a 12% decrease in the Scottish Bond Value for the xylanase-treated pulp and a 16% increase for the xylan-enriched pulp (see below).

[0042] US Patent 2015 / 0122429 describes the production of pulp from wheat straw in a four-step process. First, an organosolv digestion (in ethanolic sodium hydroxide solution at 70 °C and 18 h) was performed. The resulting pulp was then chemically (with sodium hydroxide) and enzymatically (with the commercial xylanase Pentopan BG™) dexylanized, before delignification with NaOH in the presence of Mg in the final step. 2+-ions were used. In this way, a pulp highly depleted of lignin and xylan was obtained. Sheet formation to investigate strength properties was not performed. Schaubeder et al. (2024) investigated the role of intrinsic and extrinsic xylan in a chlorine dioxide bleached (ECF) softwood kraft pulp (85% spruce, 10% pine, and 5% larch). A portion of the pulp underwent enzymatic degradation using endo-xylanase from Neocallimastix patriciarium, resulting in the degradation of approximately 1% of the xylan, and a second portion of the pulp was mixed with beechwood xylan, leading to an approximately 3% increase in xylan content. The tensile index of the paper produced showed a reduction of 8% for the enzyme-treated pulp, while an increase of 8.5% was observed for the pulp enriched with xylan. This is consistent with the results of Sjöberg et al. (2004).

[0043] The production of high-strength (tear index) cellulose sheets is of great importance. The present invention therefore aims to provide a method for producing a cellulose sheet that exhibits increased strength.

[0044] Detailed description of the invention

[0045] The task is described in a process for producing a pulp sheet with a xylan content below 25.0 wt% and with a tear index of at least 4.00 mN-m 2 -G -1dissolved, which is characterized by the combination of measures in that straw, which contains lignin, xylan and cellulose and is suspended in an aqueous phase, is treated to reduce the lignin content by means of an alkaline aqueous solution containing ethanol, thereby obtaining a first pulp, that the first pulp is treated with a xylanase to reduce the xylan content, thereby obtaining a second pulp, and that a pulp sheet is formed from the second pulp.

[0046] Contrary to conventional wisdom, reducing the xylan content in straw surprisingly results in the production of a cellulose sheet with increased strength. Previously, experts assumed that reducing the xylan content would decrease the strength of the cellulose sheet (see, e.g., Schaubeder et al. (2024)).

[0047] The invention further relates to a cellulose sheet which is obtainable according to the inventive method.

[0048] The invention further relates to a cellulose sheet with a xylan content of less than 25.0 percent by weight and a tear index of at least 4.50 mN-m 2 -G -1 The invention further relates to a cellulose sheet with a xylan content of less than 25.0 percent by weight and a tear index of at least 4.00 mN-m 2 -G -1 .

[0049] Furthermore, a cellulose sheet with a tear index of at least 4.00 mN-m is preferred. 2 -G -1 , in particular of at least 4.50 mN-m 2 -G -1 , as well as a strain at break of at least 3.5%.

[0050] The first pulp is a lignin-reduced pulp, which is subsequently referred to as "pulp 1", and the second pulp is a lignin- and xylan-reduced pulp, which is subsequently referred to as "pulp 2".

[0051] For the purposes of the present patent claims and description, pulp shall be understood to mean a suspended fibrous mass produced from plant fibers by chemical digestion, which consists predominantly of cellulose.

[0052] For the purposes of the present patent claims and description, the xylan content in weight percent is understood to be the percentage by weight of xylan in the dry mass of the solid (lignocellulose, straw, pulp).

[0053] For the purposes of the present claims and description, strength is understood to mean the so-called Tear Index, which is the tear strength per unit area mass (mN - m²). 2 -G -1 ) describes (Puls & Saake, 2010).

[0054] For the purposes of the present patent claims and description, "strain at break" is understood to be the ratio between the increase in length of a material and its original length when a tensile force is applied (unit %).

[0055] The invention further relates to a method for producing a fiber composite material, characterized by the combination of the following measures: straw containing lignin, xylan and cellulose, suspended in an aqueous phase, is treated with an alkaline aqueous solution containing ethanol to reduce the lignin content, thereby obtaining a first pulp; the first pulp is treated with xylanase to reduce the xylan content, thereby obtaining a second pulp; and a fiber composite material is formed from the second pulp.

[0056] The second cellulose described above is thus used according to the invention for the production of the fiber composite material, wherein the production of fiber composite materials from cellulose is sufficiently known to a person skilled in the art. For the purposes of the present claims and description, fiber composite material is understood to be a composite material consisting of reinforcing fibers and a matrix (filler or adhesive between the fibers). Typical fiber composite materials are, for example, paper, cardboard, or carton, which can consist of one or more different types of cellulose. These can also contain additives (e.g., fillers) or coatings.

[0057] Furthermore, the second pulp produced according to the invention can also serve as a starting material for obtaining its chemical components or as a fermentation raw material.

[0058] The invention further relates to a process for obtaining cellulose degradation products and xylan degradation products from lignocellulose, which contains lignin, xylan and cellulose, characterized by the combination of the following measures: a) the lignocellulose is treated with an aqueous solution containing a Cl-C4 alcohol and having a pH between 11 and 14, whereby a portion of the lignin contained in the lignocellulose dissolves, yielding a lignin-containing solution and a first pulp suspended in this solution; b) the first pulp is separated from the lignin-containing solution; c) the separated first pulp is treated with an aqueous solution containing xylanase to selectively reduce the xylan content in the first pulp and dissolve xylan degradation products, yielding a second pulp suspended in the xylan degradation product-containing solution.d) the second pulp is separated from the xylan-containing solution, e) the xylan-containing solution is at least partially recovered from the obtained xylan-containing solution, and f) the second pulp is hydrolyzed to cellulose-containing products, after which the cellulose-containing products are recovered.

[0059] The invention further relates to a process for obtaining pulp and xylan degradation products from straw containing lignin, xylan, and cellulose, characterized by the combination of the following measures: a) the straw is treated with an aqueous solution containing a Cl-C4 alcohol and having a pH between 11 and 14, whereby a portion of the lignin contained in the straw dissolves, yielding a lignin-containing solution and a first pulp suspended in this solution; b) the first pulp is separated from the lignin-containing solution; c) the separated first pulp is treated with an aqueous solution containing xylanase to selectively reduce the xylan content in the first pulp and dissolve xylan degradation products, yielding a second pulp suspended in the xylan degradation product-containing solution.d) the second pulp is separated from the xylan-degradation-containing solution, and e) the xylan degradation products are at least partially recovered from the obtained xylan-degradation-containing solution.

[0060] The invention also relates to a process for obtaining cellulose degradation products and xylan degradation products from straw containing lignin, xylan, and cellulose, characterized by the combination of the following measures: a) the straw is treated with an aqueous solution containing a Cl-C4 alcohol and having a pH between 11 and 14, whereby a portion of the lignin contained in the straw dissolves, yielding a lignin-containing solution and a first pulp suspended in this solution; b) the first pulp is separated from the lignin-containing solution; c) the separated first pulp is treated with an aqueous solution containing xylanase to selectively reduce the xylan content in the first pulp and dissolve xylan degradation products, yielding a second pulp suspended in the xylan degradation product-containing solution.d) the second pulp is separated from the xylan-containing solution, e) the xylan-containing solution is at least partially recovered from the obtained xylan-containing solution, and f) the second pulp is hydrolyzed to cellulose-containing products, after which the cellulose-containing products are recovered.

[0061] The invention further relates to a process for obtaining pulp, lignin, and xylan degradation products from straw containing lignin, xylan, and cellulose, characterized by the combination of the following measures: a) treating shredded straw with an aqueous solution containing a C1-C4 alcohol and having a pH between 11 and 14, whereby some of the lignin contained in the straw dissolves, yielding a lignin-containing solution and a first pulp suspended in this solution; b) separating the first pulp from the lignin-containing solution; c) recovering the lignin from the lignin-containing solution obtained; d) treating the separated first pulp with an aqueous solution containing xylanase to selectively reduce the xylan content in the first pulp and dissolve xylan degradation products, thereby obtaining a second pulp.which is suspended in the xylan-degradation-containing solution, e) the second pulp is separated from the xylan-degradation-containing solution, and f) the xylan degradation products are at least partially recovered from the obtained xylan-degradation-containing solution.

[0062] For the purposes of the present patent claims and description, lignocellulose is understood to mean biomass consisting predominantly of lignin, cellulose and hemicellulose.

[0063] The lignocellulose preferably has a lignin content of less than 35.0% by weight (based on the dry mass of the lignocellulose), and particularly preferably a lignin content of less than 26.0% by weight.

[0064] For the purposes of the present patent claims and description, straw shall be understood to mean dried fiber plants such as wheat, rye, barley, oats, spelt, millet, buckwheat, emmer, einkorn, sorghum, rice, maize, flax, grass-like raw materials such as miscanthus, sugar cane plants or components of dried fiber plants (e.g. husks), with wheat straw being particularly preferred.

[0065] For the purposes of these claims and description, enzymatic reduction of the xylan content of the pulp by treatment with xylanases (hemicellulases) is understood to mean enzymatic partial dexylanization. Thus, xylan-reduced pulp (pulp 2) is understood to be a suspended fibrous mass whose original xylan content has been reduced by the action of xylanases.

[0066] For the purposes of the present claims and description, Xylan is understood to be a polymer (polysaccharide) made from the monomer D-xylose, which, depending on its composition, may also contain L-arabinose, D-galactose, D-mannose, D-galactose, D-glucuronic acid, D-galacturonic acid as monomers.

[0067] Xylan cleavage products (as a product of enzymatic dexylanization) are to be understood, within the meaning of the present claims and description, as D-xylose, L-arabinose, xylooligosaccharides, substituted xylooligosaccharides (e.g. with arabinofuranosyl residues), uronic acids / uronates such as 4-O-methyl-D-glucuronic acid / 4-O-methyl-D-glucuronate and / or acetic acid / acetate.

[0068] For the purposes of the present claims and description, cellulose degradation products are to be understood as D-glucose, cellobiose and glucooligosaccharides.

[0069] Cellulose hydrolysis products can be obtained either through chemical or enzymatic hydrolysis (action with cellulases) of cellulose. Cellulose hydrolysis products can be obtained, for example, by concentration and / or purification.

[0070] The xylan cleavage products can be obtained from the xylan cleavage product-containing solution, e.g., by concentration and / or purification.

[0071] Lignin can be obtained from the lignin-containing solution obtained in the process, for example by precipitation or by evaporating the solution as a solid (see EP 3072117 Bl; EP 2922885 Bl; EP 3080353 Bl or EP 0970275 Bl).

[0072] The alkaline organosolv process described here, in combination with enzymatic dexylanization, offers the unique possibility of a biorefinery that combines all three options for selective product recovery: 1) the recovery of a sulfur-free and, due to the mild cooking conditions, only slightly condensed, native lignin as a chemical feedstock (see EP 2611820 Bl), 2) selective enzymatic recovery of xylan / xylose as a chemical feedstock, and 3) the production of a pulp sheet with improved strength properties, or alternatively, by hydrolysis of the second pulp sheet, the recovery of cellulose degradation products as a chemical feedstock.

[0073] A xylanase can be used in the following ways to produce xylan cleavage products, the last of which is particularly preferred: 1. Use of only endo-l,4-β-xylanase for the separation of xylan (e.g., in the form of xylooligosaccharides), 2. Use of only endo-l,4-β-xylanase for the separation of xylan and subsequent enzymatic cleavage of the xylan cleavage products to monomers (xylose and arabinose), and 3. Use of endo-l,4-β-xylanase in combination with other xylanases for the direct production of monomers.

[0074] The xylan-reduced pulp (pulp 2) produced by the action of xylanase is, after xylan extraction, in a suspension / solution which may consist of oligomers and / or monomers (xylose and arabinose) and can be used as a chemical raw material after separation of the xylan-reduced pulp.

[0075] In contrast to the process described in US 9970038 B2, in which xylose and also glucose were released from delignified wheat straw using a commercial cellulase / hemicellulase mixture, the present invention preferably uses a cellulase-free xylanase for selective dexylanization. As a result, the xylan-degradation-containing solution according to the invention contains glucose (and galactose) only in concentrations below the limit of detection (see Example 3).

[0076] Xylanase is present either in the form of a suspension, in the homogenate and / or in the lysate of the corresponding cells that produce it, or in isolated form, with homogenates being particularly preferred.

[0077] In this context, suspension refers to a suspension of resting cells. These are harvested after cultivation (separated from the nutrient medium) and suspended in a suitable buffer system. Unlike fermentative processes, which also utilize whole cells, the resting cells, due to the removal of carbon sources and nutrients, can no longer grow and serve only for substrate conversion (Lin & Tao, 2017). Homogenate, in this context, refers to a physically and / or chemically treated suspension (e.g., treated with pressure, lysozyme, or ultrasound) in which the cell components are released from the cells. A lysate is obtained when the insoluble cell components of the homogenate are removed, for example, by filtration or centrifugation.

[0078] In another variant, the enzymes can also be modified at the N-terminus with a water-soluble polymer such as polyethylene glycol, immobilized in or on a solid matrix, or be part of a fusion protein.

[0079] Regarding the tear index, it should be noted that it generally decreases with increasing refining, suggesting fiber damage during refining. To determine the optimal refining level for a given paper quality, a relationship is established between the tear index and the tensile index. However, the relevant relationship for the present invention is the connection between the xylan content of the pulp and the resulting tear index after refining.

[0080] The unmilled and lignin-reduced pulp (Table 7: Pulp 1) has a tear index of 4.61 mN-m 2 -G -1The pulp loses 26% of its strength through FPI milling at 1500 rpm and 44% at 2500 rpm. The pulps after approximately 50% removal of xylan (Table 7: Pulp 2) achieve values ​​of 4.82 mN-m^g. 1 (PFI grinding at 1500 rpm) or 4.10 mN-m 2 -G -1 again the strength range of unmilled pulp 1.

[0081] The ground lignin- and xylan-reduced pulp (pulp 2) gains 41% in tear strength (tear index) at 1500 rpm and 60% at 2500 rpm compared to the non-xylan-reduced pulp (pulp 1).

[0082] The strain at break value also increases by approximately 42% after xylanase treatment and milling at 2500 rpm compared to untreated pulp, while the burst index remains roughly the same after milling. Only the tensile index of the milled pulp decreases by 13% (PFI 1500 rpm) and 10% (PFI 2500 rpm) after xylanization.

[0083] It is clearly evident that the pulp sheet produced according to the invention, through xylanase treatment of pulp 1, experiences a significant increase in strength, measured as the Tear Index, compared to a pulp sheet made from pulp that was not treated with xylanase.

[0084] The improvement in the strength properties of the cellulose sheet through xylan extraction completely contradicts the prevailing theory regarding the function of xylan in lignocellulose fiber structures. In none of the studies cited above was the influence of enzymatic xylan extraction on the tear index measured, nor was its significant increase mentioned or acknowledged.

[0085] materials

[0086] Wheat straw was sourced from Derler Agrar GmbH, Birkfeld, Austria in October 2023. The chemical composition (see Methods for a description of the analytical methods) is shown in Table 1 below.

[0087] Hydrochloric acid, sulfuric acid, sodium hydroxide, sodium acetate, sodium dodecyl sulfate (SDS), zinc acetate and IPTG (isopropyl β-D-thiogalactopyranoside), L-arabinose and D-xylose were sourced from Carl Roth, acetone and D-galactose were sourced from AppliChem, ethanol was sourced from Honeywell, glycine, D-glucose, sodium thiosulfate, potassium permanganate and potassium hexacyanoferrate(II) trihydrate were sourced from Sigma-Aldrich and AZO-wheat-arabinoxylan, 4-nitrophenyl β-D-xylopyranoside, 4-nitrophenyl αL-arabinofuranoside and AZO-CM-cellulose were sourced from Megazyme.

[0088] Table 1. Chemical composition of the wheat straw used.

[0089] Enzymes - Methods and Analysis

[0090] General information on the expression of recombinant enzymes in E. coli

[0091] For recombinant enzyme production in an Escherichia co / / strain, the gene to be expressed was first amplified in PCR using genomic DNA or its synthetically adapted equivalent (adapted to the codon usage of E. coli) as a template, along with specific oligonucleotides that additionally carry recognition sequences for restriction endonucleases. The resulting DNA was then isolated from the reaction mixture. After nucleic acid digestion with the restriction enzymes Sphl and Hindi II, the gene fragment encoding the target enzyme was ligated into the Sphl-Hindll backbone of the expression vector pQE70-Kan. The ligation product was transformed into chemically competent E. co / / cells (ToplOF'), and the resulting colonies were used for plasmid isolation and restriction analysis.

[0092] The result of the cloning step was verified by restriction enzyme digestion and DNA sequencing. The resulting construct carries the target gene under the IPTG-inducible T5 promoter.

[0093] For the overexpression of the enzyme in E. coli, the resulting expression plasmid was transformed into competent expression cells RB791. After 24 h incubation at 37 °C, the resulting colonies were inoculated into LB medium for expression assays.

[0094] The following day, expression cultures with an optical density (OD550) of 0.02 were inoculated and shaken at 37 °C until an OD550 of 0.3 was reached. The temperature was then lowered to 25 °C, and the cultures were induced with 0.1 mM IPTG when an OD550 of 0.5 was reached. After 22 h, the cultures were harvested (separated from the medium by centrifugation into a cell pellet) and analyzed for the expression of the recombinant enzyme using SDS-gel electrophoresis and activity determination (for use in a use test or optical zymatic assay).

[0095] Production of cell homogenates using Sonifier digestion

[0096] To prepare a cell suspension, the cell pellet produced according to the above procedure was weighed into a suitable container, mixed with buffer (e.g., sodium acetate buffer), and dissolved while stirring. The biomass fraction is typically 20% by mass; the remainder consists of the buffer.

[0097] A Branson Sonifier 450 was used for cell disruption. The suspension was treated three times with 15 ultrasonic pulses each (device settings: Timer = 15; Duty Cycle = 50; Output Control = 3-5).

[0098] The resulting homogenate was used directly for the reactions.

[0099] Measurement of endo-1,4-β-xylanase activity

[0100] The activity of endo-1,4-β-xylanase was determined using the Megazyme AZO-Wheat assay. For the assay, the enzyme (E. co / / -cell homogenate) was diluted with 100 mM sodium acetate buffer (pH 5.5), and 200 pL of the enzyme solution was mixed with 200 pL of substrate solution (wheat arabinoxylan with Remazol Brilliant Blue R dye) and incubated for 10 min at 40 °C. The reaction was stopped by the addition of 1 mL of 95% ethanol and equilibrated for 10 min at room temperature. The reaction mixtures were then mixed again and centrifuged in a benchtop centrifuge at 1500 xg for 10 min. The supernatant was transferred to cuvettes, and the absorbance was measured at 590 nm. The activity was determined using a calibration curve.

[0101] Measurement of β-xylosidase activity

[0102] The β-xylosidase activity was measured using the model substrate 4-nitrophenyl-β-D-xylopyranoside (O-PNPX). 890 pL of 20 mM sodium acetate buffer (pH 5.0) were placed in a 2 mL reaction vessel and heated to 30 °C in an Eppendorf thermomixer. 10 pL of the appropriately diluted enzyme solution (E. co / / -cell homogenate) and 100 pL of substrate solution (4 mM) were pipetted to the buffer, mixed, and incubated for 5 min at 30 °C. The reaction was stopped by adding 100 pL of 500 mM glycine-NaOH buffer, the mixture was transferred to a cuvette, and the UV absorption was measured at 405 nm.

[0103] Measurement of αL-arabinofuranosidase activity

[0104] The activity of αL-arabinofuranosidase was measured using the model substrate 4-nitrophenyl-αL-arabinofuranoside (O-PNPAF). For the assay, 890 pL of 20 mM sodium acetate buffer (pH 5.0) was mixed with 10 pL of the appropriately diluted enzyme solution (E. co / / -cell homogenate) and 100 pL of substrate solution (4 mM) in a 2 mL Eppendorf vial and incubated for 5 min at 30 °C. The activity was then stopped by adding 100 pL of 500 mM glycine-NaOH buffer, the mixture was transferred to a cuvette, and the UV absorption was measured at 405 nm.

[0105] Measurement of cellulase activity

[0106] Cellulase activity was determined using the Megazyme assay for endo-1,4-beta-glucanase (cellulase) with AZO-CM-cellulose. To prepare the substrate solution, 2 g of 4M CM-cellulose stained with Remazol Brilliant Blue R were dissolved in 80 mL of boiling water with vigorous stirring. The pH was adjusted to 4.5 with 2 M sodium acetate buffer, the solution was cooled, and the volume was made up to 100 mL with water. 200 pL of the enzyme solution were incubated with 200 pL of substrate solution for 10 min at 40 °C. The reaction was then stopped by the addition of 1 mL of 96% ethanol, the mixture was thoroughly mixed, equilibrated at room temperature for 10 min, and then centrifuged at 1000 x g in a benchtop centrifuge. The supernatant was transferred to a cuvette, and the absorbance was measured at 590 nm. The evaluation was carried out using a calibration curve.

[0107] Overview of enzymes used for xylan depolymerization

[0108] Table 2. Enzyme types, donor organisms, NCBI Accession Numbers, activities and cellulase activities of the xylanases used for xylan depolymerization.

[0109] Assay: a AZO-Wheat-Arabinoxylan, b 4-Nitrophenyl-ß-D-xylopyranoside, c 4-Nitrophenyl-aL-arabinofuranoside, d AZO-CM cellulose

[0110] (see above)

[0111] To ensure that the xylanases used here exhibit no cellulase activity, the enzymes were expressed in a strain of Escherichia coli. E. coli is a bacterium for which no natural strain is known to possess the ability to enzymatically cleave cellulose. For enzyme expression, gene fragments encoding the respective target enzyme were selected. Nevertheless, in the enzyme assay, the expressed enzymes of the xylanase complex showed traces of cellulase activity (see Table 2). To demonstrate that the xylanases used do not affect the cellulose content of the xylan-reduced pulp, the amount of glucose released in the filtrate of the xylan-reduced pulp after xylan extraction was measured following the addition of the xylanase (see Example 3). This amount was below the limit of detection of the HPAEC method (see below) in all determinations.The xylanases used can therefore be described as "cellulase-free" in the sense of being ineffective in breaking down cellulose in the pulp.

[0112] Analytical methods

[0113] Determination of total sugar in solids / solutions by complete hydrolysis

[0114] Multiple determinations were performed. The solid (e.g., wheat straw, pulp 1, or pulp 2) was milled (Retsch Ultra Centrifugal Mill ZM 200) to obtain a homogeneous sample. 5 mg (± 0.1 mg) of the milled solid was weighed into a pressure test tube. For the total sugar determination from the solutions, 100–200 pL were pipetted into a pressure test tube and evaporated in a vacuum oven at 50 °C. The samples (solids or evaporated solutions) were stirred with 80 pL of 12 M sulfuric acid for 2 h using a magnetic stir bar on a magnetic stir plate at 300 rpm. The samples were then diluted with 2.75 mL of water and heated to 120 °C in a drying oven for 1 h. The test tubes were then cooled to room temperature and neutralized with calcium carbonate. The samples were centrifuged in a benchtop centrifuge for 5 minutes at maximum speed.The supernatant was removed, centrifuged again for 5 minutes, and diluted with ultrapure water. The solution was transferred to an IC vial and analyzed by HPAEC (see below).

[0115] Determination of lignin content according to Klason

[0116] For the determination of Klason lignin, 500 mg of solid were mixed with 7.5 mL of 12 M H₂SO₄ in a beaker. The mixture was stirred for 2 h at room temperature. The mixture was then transferred to an Erlenmeyer flask, the beaker was washed with hot deionized water, and the volume was increased to a total mass of 285 g. After incubation in an ultrasonic bath for 10 min, the mixture was boiled on a hot plate for 4 h, replenishing losses due to evaporation with deionized water every 30 min. The solution was then incubated overnight at room temperature. The following day, the solid was filtered through a tared glass frit and washed with hot deionized water until the wash solution was neutral. The mass of the solid (insoluble lignin fraction) was determined after drying the glass frit overnight in a vacuum drying oven. The soluble lignin fraction in the filtrate was quantified using photometry (measurement at 205 nm).Determination of the ash and silicate content in solids.

[0117] To determine the ash content, 1 g of sample (wheat straw, cellulose, or lignin) was weighed into a previously annealed and tared ceramic crucible and ashed with a Bunsen burner until no more smoke was visible. The crucible was then heated overnight in a muffle furnace (575 °C). The mass of the ash was determined by weighing.

[0118] To determine the silicate content, the entire quantity of ash was placed in a beaker and mixed with 5 mL of 6 M HCl. The mixture was then heated on a hot plate (200–220 °C). After the HCl had evaporated, another 5 mL of 6 M HCl was added and evaporated again. This process was repeated (total addition: 15 mL of 6 M HCl). Subsequently, 20 mL of deionized water were added, and the solid was filtered through an ash-free filter. The filtrate was then washed with hot deionized water until it reached pH neutral. After ashing the filter, the silicate content was determined by weighing.

[0119] Determination of the proportion of extractable substances by means of Soxhlet extraction

[0120] Four grams of shredded wheat straw were weighed into a pre-dried and weighed cellulose extraction pod. Extraction was carried out in a Soxhlet apparatus for two hours with 500 mL of acetone. The extraction pod was then dried overnight in a vacuum drying oven, and the proportion of extractables was determined as the mass difference.

[0121] Determining the kappa number

[0122] 30 mg of a solid (e.g., cellulose) were weighed into a 50 mL Erlenmeyer flask, mixed with 14 mL of water, and stirred until a homogeneous suspension was formed. Then, 2 mL of a 0.1 N potassium permanganate solution were added. After 10 min, 400 µL of a 10% potassium permanganate solution were added, resulting in an orange-brown suspension. This was titrated with a 0.1 N sodium thiosulfate solution until a pale yellow solution was obtained. To improve visibility of the endpoint, 200 µL of starch solution were added, and the titration was continued until complete decolorization. The kappa number was calculated from the consumption of the sodium thiosulfate solution.

[0123] Determination of the grind size according to Schopper-Riegler

[0124] Two grams of sample and one liter of distilled water were placed in the container of the mixing device (Frank-PTI) and mixed until the desired number of revolutions (30,000 revolutions) was reached. The suspension was then transferred to the Schopper-Riegler grinding tester (Frank-PTI), where it was separated using a sieve. The Schopper-Riegler value (°SR) was read on the scale of the measuring cup.

[0125] Sample preparation for HPAEC analysis using Carrez precipitation

[0126] For Carrez solution I, 21.9 g of zinc acetate were dissolved in 100 mL of water. Carrez solution II was prepared by dissolving 10.6 g of potassium hexacyanoferrate(II) in 100 mL of water.

[0127] 70 pL of Carrez solution I were pipetted to 1 mL of sample (filtrate or wash solution after enzyme treatment) and incubated at 25 °C and 1000 rpm for 5 min. Then, 70 pL of Carrez solution II were added and incubated for another 5 min at 25 °C and 1000 rpm. The resulting precipitate was then separated by centrifugation in a benchtop centrifuge for 10 min at 1000 rpm. Further dilution was performed, if necessary, before HPAEC measurement.

[0128] High Performance Anion Exchange Chromatography (HPAEC)

[0129] A Dionex ICS6000 system with an AS-AP autosampler was used to quantify arabinose, glucose, galactose, and xylose by HPAEC (High Performance Anion Exchange Chromatography). A Dionex CarboPac PA20-fast-4pm column with a suitable guard column and a NaOH gradient was used for analyte separation. The analytes were detected using a pulsed amperometric detector (PAD, gold electrode) and the "Carbo, Quad" waveform.

[0130] The following examples describe preferred variants of the method according to the invention in more detail.

[0131] Example 1

[0132] Reduction of the lignin content of straw

[0133] 2.4 kg of chopped, sieved wheat straw (particle size 1–5 cm; composition see Table 4) were suspended in 7.2 L of a reaction solution consisting of 2.72 kg water, 4.03 kg ethanol, and 0.27 kg NaOH in a horizontal paddle reactor (total volume: 80 L). The suspension was heated to 120 °C in the reactor (heating time to reach 120 °C was approximately 15–20 min) and stirred at 11 rpm for 15 min. The reaction vessel was then cooled to approximately 40 °C, the suspension was emptied, and diluted to 4% density with water.

[0134] The suspension was then refined in a refiner (Sprout-Bauer 12" (7208-110); 0.3 mm plate spacing), during which the fibers were mechanically processed. The solid fraction was separated from the aqueous solution using a washing press (Fischer stainless steel hydropress; 90 L working volume), yielding a lignin-containing aqueous solution and a lignin-reduced pulp. The lignin-reduced pulp was washed three times with 20 L of water using the washing press. The lignin-reduced pulp was then further washed with water through sieves (80 µm mesh size) until the pH of the wash solution was < 9 and treated with a spin dryer (Thomas Centri 776 SEK) to increase the dry matter content to approximately 30%. Finally, the lignin-reduced pulp was blended using a kneading machine (GGM Gastro PRMH20; 20 L working volume).The pulp obtained in this way with reduced lignin content is referred to below as "pulp 1 (= first pulp)".

[0135] Table 3 below shows the dry matter, yield, kappa number, and Schopper-Riegler value of pulp 1. Table 4 below lists the cellulose, xylan, and lignin contents of wheat straw and pulp 1.

[0136] Table 3. Characterization of cellulose 1.

[0137] Table 4. Cellulose (corresponds to the glucose content), xylan (sum of xylose, arabinose and galactose)

[0138] Content) and lignin content (according to Klason) of wheat straw and the pulp obtained from wheat straw 1.

[0139] The data in Table 4 show a significant reduction in lignin content (Klason) from 22.4% to 7.7%.

[0140] Example 2

[0141] Lignin precipitation

[0142] 500 mL of the lignin solution obtained in Example 1 were transferred to a round-bottom flask. The ethanol contained in the solution was evaporated using a rotary evaporator at a water bath temperature of < 40 °C. After determining the lignin concentration (21.4 g / L) by photometry (measurement at 280 nm in 10 mM NaOH), the solution was diluted to a lignin concentration of 10 g / L with deionized water.

[0143] A duplicate determination was performed using 200 mL of the diluted lignin solution (10 g / L) in each sample. The pH of the solutions was first adjusted to < 4 with 66% H₂SO₄ while stirring. The solutions were then incubated without stirring in a water bath at 65 °C for 60 min, during which time a solid formed. Immediately after incubation, the solid was filtered off using AllS filter paper in a Büchner funnel and washed with a small amount of deionized water.

[0144] In this way, 1.4 g of solid (70% yield) were obtained. The sugar content and lignin content (Klason) of the recovered solid are shown in Table 4 (average from the duplicate determinations).

[0145] Table 5. Sugar content and lignin content (Klason) of the precipitated solid.

[0146] The analysis shows that the obtained solid consists of 91% lignin and has a very low proportion of xylose and arabinose.

[0147] Example 3

[0148] Reduction of the xylan content

[0149] The xylan of the pulp 1 produced in Example 1 was enzymatically depolymerized and the solution containing the xylan degradation products was separated from the solid (pulp 2, = "second pulp").

[0150] The reaction was carried out in a spherical 1 L glass reaction vessel with a lid, heated by a magnetic stirrer and heat-on attachment, and equipped with a KPG stirrer and a stirring shaft with a crescent-shaped stirring blade. The reaction volume was 400 mL. 40 g (dry mass) of pretreated substrate (pulp 1) was suspended in water, and the pH was adjusted to 6 (±0.2) with 1 M sulfuric acid. After stabilizing the pH, the xylanases were added in the form of E. coli cell homogenates (10.7 mL endo-1,4-β-xylanase I, 10 mL endo-1,4-β-xylanase II, 5 mL β-xylosidase, 10 mL αL-arabinofuranosidase).

[0151] The suspension was thoroughly mixed and incubated for 6 h at 50 °C and 50 rpm. To deactivate the enzymes, the suspension was heated to 95 °C for 15–30 min. After cooling to approximately 40 °C, the solution was separated from the solid by filtration through a Büchner funnel, and the enzyme-treated pulp was washed twice with 200 mL of water each time.

[0152] The sugar content of the substrate pulp (pulp 1), the enzyme-treated pulp (pulp 2), as well as the total sugar concentration and sugar monomer concentration of the filtrate and the washing solutions were analyzed using HPAEC.

[0153] The concentrations of monomeric pentoses obtained after enzyme treatment in the filtrate of pulp 2 averaged 2.7 g / L arabinose and 22.8 g / L xylose. The concentrations of the monomeric hexoses galactose and glucose were below the limit of detection in all determinations, indicating negligible cellulase activity of the enzymes used (see Table 2 for the cellulase activities of the xylanases).

[0154] Table 6 below shows the sugar content in lignin-reduced pulp (pulp 1) and in lignin- and xylan-reduced pulp (pulp 2). The values ​​shown are the result of multiple determinations – pulp 1: mean of 16 samples (4 quadruple determinations) and pulp 2: mean of 10 samples (5 duplicate determinations).

[0155] Table 6. Sugar content of pulp 1 (lignin-reduced) and pulp 2 (lignin- and xylan-reduced).

[0156] * The average yield of pulp 2 (82%) was included in the calculation of the stated percentage changes.

[0157] A significant decrease in the pentoses arabinose and xylose is observed in pulp 2 (-59.2% and -54.3% compared to pulp 1, respectively), while the hexoses glucose and galactose (+0.5% and +9.5% compared to pulp 1, respectively) are enriched. The xylan content (sum of the proportions of the xylan monomers arabinose and xylose) decreases from 36% (pulp 1) to 20% (pulp 2).

[0158] The xylan degradation products arabinose and xylose can be obtained from the solution by concentration and / or purification. Example 4

[0159] Formation of cellulose sheets

[0160] 30 g of pulp 1 (lignin-reduced; produced in Example 1) or pulp 2 (lignin- and xylan-reduced; produced in Example 3) were milled at a solids concentration of 1.5% in water in a PFI mill according to standard ISO 5264-2:2011 at 1500 rpm and 2500 rpm (PFI 1500 or PFI 2500), and paper sheets were produced to determine the strength values. Sheet formation was carried out according to standard ISO 5269-2:2004 on a sheet former from Frank-PTL.

[0161] The strength values ​​were determined according to the following standards: Tear Index (ISO 1974:2012), Strain at Break (ISO 1924-2:2009), Tensile Index (ISO 1924-2:2009), and Burst Index (ISO 2758:2014). The measurement results are shown in Table 7 below, as well as in Figures 1 (Tear Index) and 2 (Strain at Break), where 0 represents unmilled, PFI 1500 represents milling at 1500 rpm, and PFI 2500 represents milling at 2500 rpm.

[0162] Table 7. Strength parameters of sheets produced from pulp 1 (lignin-reduced) and pulp 2 (lignin- and xylan-reduced). PFI 1500 represents milling with a PFI mill at 1500 rpm and PFI 2500 represents milling at 2500 rpm. The percentages in parentheses describe the percentage change in strength values ​​after xylanase treatment.

[0163] Table 7 shows that despite the decrease in the xylan polymer in pulp 2, its strength values ​​(expressed as tear index) surprisingly increase (see percentage increases in the table). Example 5

[0164] Extraction of cellulose decomposition products from pulp 2 (straw)

[0165] The reaction was carried out in a spherical 1 L glass reaction vessel with a lid, heated by a magnetic stirrer and heat-on attachment, and equipped with a KPG stirrer and a stirring shaft with a crescent-shaped stirring blade. The reaction volume was 420 mL. 42.2 g (dry mass) of cellulose 2 (prepared in Example 3) were suspended in water, and the pH was adjusted to 6 (±0.2) with 1 M sulfuric acid. After stabilizing the pH, 16 mL of a cellulase mixture (Sigma-Aldrich SAE00200; "CellicCTec2") was added.

[0166] The suspension was thoroughly mixed and incubated for 72 h at 50 °C and 50 rpm. To deactivate the enzymes, the suspension was heated to 95 °C for 15–30 min. After cooling to approximately 40 °C, the solution was separated from the solid by filtration through a Büchner funnel.

[0167] The filtrate (330 mL) contained 0.5 g arabinose, 0.1 g galactose, 29.7 g glucose and 6.2 g xylose.

[0168] The cellulose degradation products contained in the filtrate can be obtained by concentration and / or purification.

[0169] Example 6

[0170] Extraction of a xylan-containing solution, lignin, and pulp from oat hulls

[0171] Reduction of the lignin content of oat hulls

[0172] 21.6 g of oat hulls (lignin: 33 wt%; xylan: 31.2 wt%; cellulose: 32.8 wt%; composition see Table 8) were treated in a Büchiglasuster picoclave pressure reactor with 2.1 g of NaOH dissolved in 45.9 mL of deionized water and 63.7 mL of EtOH (96 v%). The reaction mixture was heated to 120 °C and this temperature was maintained for 60 min. After cooling the reactor to 50 °C, the pulp was separated from the dark brown lye (lignin-containing solution) by filtration and washed with 3 x 300 mL of deionized water.

[0173] In this way, 39.3 g of wet pulp 1 (equivalent to 14.4 g of absolutely dry pulp) were obtained. The sugar content and lignin content (Klason) of the pulp are given in Table 8. Table 8. Cellulose (corresponds to the glucose content), xylan (sum of xylose, arabinose, and galactose)

[0174] Content) and lignin content (according to Klason) of oat hulls and the pulp obtained from oat hulls 1.

[0175] The data in Table 8 show a significant reduction in lignin content (Klason) from 33.0% to 5.7%.

[0176] Delignification was repeated to provide sufficient solution for lignin precipitation.

[0177] Reduction of the xylan content

[0178] The reaction was carried out in a spherical 1 L glass reaction vessel with a lid, heated by a magnetic stirrer and heat-on attachment, and equipped with a KPG stirrer and a stirring shaft with a crescent-shaped stirring blade. The reaction volume was 140 mL. 13.3 g (dry mass) of cellulose 1 were suspended in water, and the pH was adjusted to 6 (±0.2) with 1 M sulfuric acid. After stabilizing the pH, hemicellulases in the form of E. co / / -cell homogenates (1.1 mL endo-1,4-β-xylanase 1, 1 mL endo-1,4-β-xylanase II, 0.5 mL β-xylosidase, 1 mL αL-arabinofuranosidase) were added.

[0179] The suspension was thoroughly mixed and incubated for 22 h at 50 °C and 50 rpm. To deactivate the enzymes, the suspension was heated to 95 °C for 15–30 min. After cooling to approximately 40 °C, the solution was separated from the solid by filtration through a Büchner funnel, and the enzyme-treated pulp (pulp 2) was washed twice with 100 mL of water each time.

[0180] The filtrate and the two washing solutions were combined. The combined solution (378 mL) contained 0.26 g of arabinose and 3.2 g of xylose.

[0181] Table 9 below shows the xylan and cellulose contents of pulp 1 and 2. Table 9. Xylan and cellulose contents of pulp 1 (lignin-reduced) and pulp 2 (lignin- and xylan-reduced).

[0182] Table 9 shows a significant reduction in xylan (sum of the monomers xylose and arabinose) from 38.7 to 21.5 percent by weight due to enzyme treatment.

[0183] The xylan cleavage products arabinose and xylose can be obtained from the solution by concentration and / or purification.

[0184] Lignin precipitation

[0185] 1.8 L of the lignin-containing solution was transferred to a round-bottom flask. The ethanol contained in the solution was evaporated using a rotary evaporator at a water bath temperature of < 40 °C. Photometric determination (measurement at 280 nm in 10 mM NaOH) yielded a lignin concentration of 6.7 g / L.

[0186] 500 mL of the ethanol-free solution were placed in a beaker. The pH of the solution was first adjusted to 4.85 with 66% H₂SO₄ while stirring. The solution was then incubated without stirring in a water bath at 65 °C for 60 min, during which time a solid formed. Immediately after incubation, the solid was filtered through A113 filter paper in a Büchner funnel and washed with a small amount of deionized water.

[0187] In this way, 1.1 g of lignin could be obtained as a brown solid.

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Claims

Patent claims 1. Process for producing a pulp sheet with a xylan content of less than 25.0% by weight and with a tear index of at least 4.00 mN-m 2 -G -1 , characterized by the combination of measures in which straw containing lignin, xylan and cellulose, suspended in an aqueous phase, is treated to reduce the lignin content by means of an alkaline aqueous solution containing ethanol, thereby obtaining a first pulp, in which the first pulp is treated with a xylanase to reduce the xylan content, thereby obtaining a second pulp, and in which a pulp sheet is formed from the second pulp.

2. Cellulose sheet obtainable by a method according to claim 1.

3. Pulp sheet with a xylan content below 25.0% by weight and with a tear index of at least 4.00 mN-m 2 -G -1 .

4. Pulp sheet according to one of claims 2 or 3 having a tear index of at least 4.50 mN-m 2 -G -1 .

5. Pulp sheet according to any one of claims 2 to 4 having a strain at break of at least 3.5%.

6. A process for producing a fiber composite material, characterized by the combination of the following measures: treating straw containing lignin, xylan, and cellulose, suspended in an aqueous phase, to reduce the lignin content by means of an alkaline aqueous solution containing ethanol, thereby obtaining a first pulp; treating the first pulp with xylanase to reduce the xylan content, thereby obtaining a second pulp; and forming a fiber composite material from the second pulp.

7. Process for obtaining cellulose degradation products and xylan degradation products from Lignocellulose, which contains lignin, xylan and cellulose, is characterized by the A combination of measures comprising: a) treating the lignocellulose with an aqueous solution containing a Cl-C4 alcohol and having a pH between 11 and 14, causing some of the lignin contained in the lignocellulose to dissolve and yielding a lignin-containing solution and a first pulp suspended in this solution; b) separating the first pulp from the lignin-containing solution; c) treating the separated first pulp with an aqueous solution containing xylanase to selectively reduce the xylan content in the first pulp and dissolve xylan degradation products, yielding a second pulp suspended in the xylan degradation product-containing solution; d) separating the second pulp from the xylan degradation product-containing solution; e) at least partially recovering the xylan degradation products from the resulting xylan degradation product-containing solution.and f) that the second pulp is hydrolyzed to cellulose degradation products, after which the cellulose degradation products are obtained.

8. A process for obtaining pulp and xylan degradation products from straw containing lignin, xylan, and cellulose, characterized by the combination of the following steps: a) treating the straw with an aqueous solution containing a Cl-C4 alcohol and having a pH between 11 and 14, causing some of the lignin contained in the straw to dissolve and yielding a lignin-containing solution and a first pulp suspended in this solution; b) separating the first pulp from the lignin-containing solution; c) treating the separated first pulp with an aqueous solution containing xylanase to selectively reduce the xylan content in the first pulp and dissolve xylan degradation products, yielding a second pulp suspended in the xylan degradation product-containing solution. d) the second pulp is separated from the xylan-containing solution, and e) the xylan-containing solution is at least partially recovered from the xylan-containing solution.

9. A process for obtaining cellulose hydrolysis products and xylan hydrolysis products from straw containing lignin, xylan, and cellulose, characterized by the combination of the following steps: a) treating the straw with an aqueous solution containing a Cl-C4 alcohol and having a pH between 11 and 14, causing some of the lignin contained in the straw to dissolve and yielding a lignin-containing solution and a first pulp suspended in this solution; b) separating the first pulp from the lignin-containing solution; c) treating the separated first pulp with an aqueous solution containing xylanase to selectively reduce the xylan content in the first pulp and dissolve xylan hydrolysis products, yielding a second pulp suspended in the xylan hydrolysis product-containing solution; d) separating the second pulp from the xylan hydrolysis product-containing solution. is separated,e) that the xylan cleavage products are at least partially recovered from the obtained xylan-containing solution, and f) that the second pulp is hydrolyzed to cellulose cleavage products, after which the cellulose cleavage products are recovered.

10. A process for obtaining pulp, lignin and xylan degradation products from straw containing lignin, xylan and cellulose, characterized by the combination of measures in that a) shredded straw is treated with an aqueous solution containing a C1-C4 alcohol and having a pH between 11 and 14, whereby some of the lignin contained in the straw dissolves and a lignin-containing solution and a first pulp suspended in this solution are obtained, b) the first pulp is separated from the lignin-containing solution, c) the lignin is obtained from the lignin-containing solution, d) the separated first pulp is treated with an aqueous solution containing a xylanase to selectively reduce the xylan content in the first pulp and to dissolve xylan degradation products, thereby obtaining a second pulp suspended in the xylan degradation product-containing solution, e) the second pulp is separated from the xylan degradation product-containing solution, and f) the xylan degradation products are at least partially recovered from the xylan degradation product-containing solution obtained.

11. Method according to claim 7, characterized in that the lignocellulose has a lignin content of less than 35.0 percent by weight.

12. The method of claim 7, characterized in that the lignocellulose comprises a lignin- has a content below 26.0 percent by weight.

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