Modified lignin and method for lignin modification

Abstract

The present invention is directed to modified lignin having a decreased tendency to swell when heated. The invention is also directed to a method for modifying lignin to achieve the decreased swelling tendency. In the method lignin in solid form is provided and immersed in an acidic aqueous solution having a pH below 1.0, wherein the temperature of the acidic aqueous solution during the immersion is in the range of from 90°C to 130°C, until the swelling tendency of the lignin, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, is in the range of from 0.8 to 3.0, to obtain modified lignin that is separated from the acidic aqueous solution.

Description

[0001] MODIFIED LIGNIN AND METHOD FOR LIGNIN MODIFICATION

[0002] Field of the invention

[0003] The present invention is directed to modified lignin having a decreased tendency to swell when heated. The invention is also directed to a method for modifying lignin to achieve the decreased swelling tendency.

[0004] Background

[0005] Lignin, an aromatic polymer, is a major constituent in e.g. wood, and is the most abundant carbon source on Earth second only to cellulose. In recent years, with development and commercialization of technologies to extract lignin in a highly purified, solid and particularized form from the pulp-making process, it has attracted significant attention as a possible renewable substitute to primarily aromatic chemical precursors currently sourced from the petrochemical industry.

[0006] Today, the most commercially relevant source of lignin is kraft lignin. This lignin is dissolved from hardwood or softwood through the kraft process, resulting in residual black liquor. The lignin can be separated from alkaline black liquor using for example membrane- or ultrafiltration. LignoBoost is one common separation process and is described in for example W02006031175 A1. In this process, lignin is precipitated from alkaline black liquor through reducing the pH level, usually by adding carbon dioxide, and then filtered off. The lignin filter cake is in the next step re-slurried under acidic conditions, commonly using sulfuric acid, and washed. The precipitated washed lignin can be used as it is or further dried.

[0007] Lignin may also be obtained through different fractionation methods such as an organosolv process or hydrolysis lignin. The organosolv process is however of less commercial interest for producing lignin compared to the kraft process. When lignin is converted to carbon enriched materials that in turn is to be used in energy storage applications, the lignin needs to be subjected to heating, to achieve carbonization. Such heating is typically carried out stepwise. A well-known problem with lignin is its tendency to expand upon heating, i.e. the volume of the lignin increases when the lignin is subjected to heating. Such volume expansion (swelling) typically causes handling problems.

[0008] Thus, there is a need for lignin that is easier to handle during heating, in particular a lignin that has a reduced tendency to swell during heating.

[0009] Summary of the invention

[0010] It is an object of the present invention to provide a lignin with improved properties compared to prior art lignins, in particular a lignin having reduced tendency to swell when heated.

[0011] It has surprisingly been found that a lignin with surprisingly reduced tendency to swell when heated can be obtained by the method of the present invention. The modified lignin obtained is easier to process and has improved filtration properties, i.e. the modified lignin can be filtered from water in a shorter time than unmodified lignin.

[0012] According to a first aspect, the modified lignin according to the present invention can be prepared by a method comprising the following steps: a) providing lignin in solid form; b) preparing a slurry comprising the lignin in an acidic aqueous solution, wherein the slurry has a pH below 1 .0, and treating the slurry at a temperature of from 90°C to 130°C for a duration t in order to obtain said modified lignin; and c) separating the obtained modified lignin from the acidic aqueous solution.

[0013] The lignin remains in solid form during all steps of the method.

[0014] The slurry may comprise, consist essentially of, or consist of the lignin in an acidic aqueous solution.

[0015] For example, the modified lignin may be prepared by a method comprising the following steps: a) providing lignin in solid form; b) providing an acidic aqueous solution having a pH below 1.0; c) immersing the lignin in the acidic aqueous solution, wherein the temperature of the acidic aqueous solution during the immersion is in the range of from 90°C to 130°C, until the swelling tendency of the lignin, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, is in the range of from 0.8 to 3.0, to obtain said modified lignin; d) separating the obtained modified lignin from the acidic aqueous solution; and e) optionally washing the separated modified lignin; wherein the lignin remains in solid form during all steps of the method.

[0016] Thus, in a second aspect, the present invention is also directed to the modified lignin as such.

[0017] The modified lignin may be characterized in having a swelling tendency, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, in the range of from 0.8 to 3.0. Alternatively, or in addition, the modified lignin may be characterized in having a concentration of aliphatic hydroxy groups of less than or equal to 1 .4 mmol / g, such as less than or equal to 1 .3 mmol / g, such as less than or equal to 1 .2 mmol / g. The concentration of aliphatic hydroxy groups is determined using quantitative31P NMR by the method described in Granata et al. (Journal of Agriculture and Food Chemistry, 1995, 43, 1538-1544) but using encto-HNDI as internal standard (Holzforschung 2001 , 55, 283-285).

[0018] Alternatively, or in addition, the modified lignin may be characterized as having an average pore size, as determined by N2 BET, of less than 50 A, such as less than 40 A, such as less than 30 A, such as less than 20 A, such as less than 10 A.

[0019] A lignin material with reduced swelling tendency is of interest in a number of different applications, particularly for conversion to carbon enriched materials such as carbon fibers and carbon powders, in particular for use in energy storage applications. In such applications, the reduced swelling tendency is desirable since it is particularly beneficial for the subsequent conversion, such as by carbonization, to carbon enriched materials such as carbon fibers and carbon powders, in particular for use in energy storage applications. The present invention is also directed to a carbon enriched material produced from the modified lignin according to the present invention.

[0020] In a further aspect, the invention is directed to a method for preparing a lignin-carbon composite. Such a method comprises the following steps: a) providing lignin in solid form; b) providing a carbon powder; c) preparing a slurry comprising the lignin and the carbon powder in an acidic aqueous solution, wherein the slurry has a pH below 1 .0, and treating the slurry at a temperature of from 90°C to 130°C for a duration t in order to obtain said lignin-carbon composite; and d) separating the obtained lignin-carbon composite from the acidic aqueous solution.

[0021] The lignin remains in solid form during all steps of the method.

[0022] The slurry may comprise, consist essentially of, or consist of the lignin and carbon powder in an acidic aqueous solution. The combined dry weight of the lignin plus carbon powder relative to the total weight of the slurry may be from about 10 wt% to about 30 wt%, such as from about 15 wt% to about 25 wt%. The ratio by weight of lignin to carbon in the slurry may be from about 1 :1 to about 99:1 , such as from about 3:1 to about 49:1 , such as from about 4:1 to about 19:1.

[0023] Unless otherwise stated, all features disclosed in relation to the method as defined in the first aspect are equally applicable to the method in accord with this further aspect.

[0024] Such a method may provide a means of utilizing a carbon powder that has limited other uses. For example, in processes for producing carbon from lignin, a sizeable fraction of the obtained carbon is rejected as having too small particle size. The present method provides a convenient means of recycling such fines material.

[0025] The carbon powder may be any suitable carbon powder, including, but not limited to, soft carbon, hard carbon, graphitic carbon, and combination thereof. Preferably, the carbon powder is carbon obtained from the partial or complete carbonization or calcination of a biomass precursor, such as a lignocellulosic biomass, cellulosic biomass, lignin, or combinations thereof.

[0026] In a further step, the lignin-carbon composite may be converted into a carbon powder, and a first size fraction of this carbon powder may be recycled as the carbon powder supplied in step b) of the process. Means of converting lignin- carbon composites into carbon powders are disclosed i.a. in WO2023105438 and WO2024252231 . In yet another aspect, the invention is directed to a lignin-carbon composite obtainable by the method above. The lignin-carbon composite may have a ratio by weight of lignin to carbon of from about 1 : 1 to about 99: 1 , such as from about 3: 1 to about 49: 1 , such as from about 4: 1 to about 19:1.

[0027] Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

[0028] Brief description of the drawings

[0029] For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

[0030] Fig 1a shows a SEM image of an untreated lignin sample;

[0031] Fig. 1 b shows a SEM image of a modified lignin sample in accord with the present invention;

[0032] Fig. 2 shows the results of a DoE study as a plot of the determined coefficients (scaled and centered) for the decrease in swelling tendency relative to a reference lignin;

[0033] Fig. 3 shows a plot of predicted decrease in swelling relative to the experimentally observed values obtained in the DoE study;

[0034] Fig. 4 shows the the swelling tendency for a number of sample lignins periodically obtained during progression of the reaction in the kilogram-scale study.

[0035] Detailed description

[0036] Step a) of the method for preparing the modified lignin according to the present invention involves providing lignin in solid form. It is intended throughout the present description that the expression "lignin" embraces any kind of lignin, e.g. lignin originated from hardwood, softwood or annular plants. Also, lignin can be chemically modified. Preferably, the lignin has been modified or isolated before being used in the process according to the present invention. The lignin may be isolated from black liquor and optionally be further modified before being used in the process according to the present invention. The purification is typically such that the purity of the lignin material is at least 90%, preferably at least 95%, more preferably at least 98%, based on the dry weight of the lignin material, Thus, the lignin material used according to the process of the present invention preferably contains less than 10%, preferably less than 5%, more preferably less than 2% impurities, such as cellulose and inorganic compounds, based on the dry weight of the lignin material.

[0037] The lignin to be modified may be obtained through different separation methods such as an organosolv process or a Kraft process or through biorefinery methods. Preferably, the lignin provided in step a) of the method for preparing the modified lignin according to the present invention is Kraft lignin, i.e. lignin obtained through the Kraft process. Kraft lignin has been found to be especially prone to swelling upon thermal treatment. Preferably, the Kraft lignin is obtained from hardwood or softwood, most preferably from softwood. More specifically, it is preferred that at least 50 wt-%, such as at least 60 wt-% or 70 wt-% or 80 wt-% or 90 wt-% or 95 wt-% or 98 wt-% or 100 wt-% of the wood used to produce the lignin is from softwood.

[0038] The lignin to be modified may be obtained by using the process disclosed in W02006031 175 A1 commonly referred to as the LignoBoost process. Typically, this process involves the steps of precipitation of lignin from alkaline black liquor by acidification; separation of the precipitated lignin; and re-slurrying the lignin under acidic conditions, preferably at a temperature in the range of from 40°C to 65°C, such as from 50° to 60°C, such as at approximately 55°C at least once. The obtained lignin may be dried and pulverized and thus provided as solid particles. The lignin obtained by this method typically has a total metal content in the range of from 500 ppm to 5000 ppm, originating mainly from the wood source and cooking chemicals added during the pulping process. The pH of the obtained lignin is typically in the range of from 3 to 4. The sulfur content of the obtained lignin is typically in the range of from 1 to 3 wt-%. This lignin is the preferred starting material to obtain the modified lignin of the present invention.

[0039] The lignin provided in step a) of the method for preparing the modified lignin according to the present invention is in solid form, i.e. it is not in a completely dissolved state. In one embodiment, the lignin is provided in the form of a dry powder. Alternatively, the lignin may be moist or provided in a slurry or suspension. The lignin may also be provided as a crushed lignin cake obtained from a lignin separation process. The lignin is preferably not dissolved during any of the steps in the method for preparing the modified lignin according to the present invention but remains in solid form.

[0040] Alternatively, the lignin may be dissolved to a small extent, such that only small fragments of the solid lignin is dissolved, during the steps of the method according to the present invention. Thus, the lignin will remain largely in solid state. Any dissolved lignin will be removed during the separation step and discarded from the process.

[0041] Preferably, the lignin provided in step a) of the method for preparing the modified lignin according to the present invention is in particulate form, such as in the form of a powder. Particle size distribution of such particles can be determined by laser diffraction using a Malvern Mastersizer 3000. For such measurement, a dry powder lignin sample is sonicated for 120 seconds before the analysis. The lignin sample is dispersed and conveyed from the sample hopper to the laser diffractometer in pressurized air. A refractive index of 1 .59 is used for calculating the particle size. The particle size distribution of the lignin particles is preferably such that dx50 (= dv50) of the particles is less than 12 pm. In the context of the present invention, the diameter of a particle is the equivalent spherical diameter of the particle, if the particle is not spherical. The equivalent spherical diameter is the diameter of a sphere of equivalent volume. Step b) of the method for preparing the modified lignin according to the present invention involves providing an acidic aqueous solution having a pH below 1 .5. The term “acidic aqueous solution” as used herein, refers to any type of aqueous solution having a pH below 1 .5. The acidic aqueous solution in step b) of the method for preparing the modified lignin according to the present invention may be provided by adding at least one acid to an aqueous solution. The pH of the acidic aqueous solution provided in step b) of the method for preparing the modified lignin according to the present invention has a pH below 1 .5, preferably below 1 .2 or below 1 .0 or below 0.8. and more preferably below 0.7.

[0042] Lignin may be added to the aqueous solution after the acid has been added. Alternatively, lignin may be added to the aqueous solution prior to adding the acid. The aqueous solution is heated either before or after addition of the acid and lignin. In one embodiment, the acid is added to a heated aqueous solution, to which lignin is subsequently added. In another embodiment, lignin is added to an acidic aqueous solution which is subsequently heated. In yet another embodiment, acid is added to an aqueous solution comprising lignin. In this embodiment, the aqueous solution is heated either before or after addition of acid. In a preferred embodiment, lignin is added to a heated aqueous solution, and acid is subsequently added to the aqueous solution comprising lignin.

[0043] The acidic aqueous solution comprises at least one acid such that the pH of the acidic aqueous solution is below 1 .5, preferably below 1 .2 or below 1 .0 or below 0.8, or below 0.7 or below 0.5. Examples of suitable acids are sulfuric acid, phosphoric acid, nitric acid and hydrochloric acid.

[0044] The acidic aqueous solution may also comprise more than one acid, such as a combination of two or more acids. In one embodiment of the method for preparing the modified lignin according to the present invention, the total amount of acid in the acidic aqueous solution is in the range of from 7 to 30 wt%, preferably from 10 to 24 wt%, based on the dry weight of lignin immersed in the acidic aqueous solution. The term “total amount of acid” as used herein, refers to the total amount of concentrated acid added to the acidic aqueous solution.

[0045] The acidic aqueous solution may further comprise one or more additives.

[0046] Step c) of the method for preparing the modified lignin according to the present invention involves immersing the lignin particles in the acidic aqueous solution, wherein the temperature of the acidic aqueous solution during the immersion is in the range of from 80°C to 130°C, until the swelling tendency of the lignin, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, is in the range of from 0.8 to 3.0, to obtain said modified lignin.

[0047] The term “immersion” as used herein, refers to a process of contacting lignin in solid form, such as in the form of lignin particles, with an acidic aqueous solution for a certain period of time. During the immersion step of the method for preparing the modified lignin according to the present invention, the entire surface area of the lignin is in contact with the acidic aqueous solution, meaning that the lignin is fully submerged in the acidic aqueous solution.

[0048] The lignin is immersed in the acidic aqueous solution at a temperature in the range of from 80°C to 130°C, such as from 90°C to 130°C, such as from 100°C to 130°C, such as from 101 °C to 129°C, or from 110°C to 128°C or from 111 °C to 126°C. The acidic aqueous solution may be heated using any suitable means as known by a person skilled in the art. If needed, the heating is carried out in a closed pressurized vessel. The temperature is kept in the defined range during the entire immersion step, but is not necessarily constant during the duration of the step. In one embodiment, the temperature is increased step-wise, such that the aqueous solution is initially at a temperature in the range of from 80°C to 100°C and subsequently the temperature is increased to a temperature in the range of from 100°C to 130°C. It has been found that increasing the temperature slowly or step-wise leads to a modified lignin that has particularly desirable properties and avoids lump formation of the lignin.

[0049] It has been found that treating the lignin slurry at a lower pH permits use of lower reaction temperatures. The slurry may have a pH below 0.7. In such a case, the slurry may be treated at a temperature of from 90 °C to 110 °C, preferably from 90 °C to 100 °C. Treatment at lower temperatures permits use of reaction vessels that do not have to withstand elevated pressures, thus facilitating industrial implementation.

[0050] The immersion in step c) of the method for preparing the modified lignin according to the present invention is carried out until the swelling tendency of the lignin, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, is in the range of from 0.8 to 3.0. Typically, the immersion time is at least 30 minutes, preferably at least 60 minutes, or even more preferably at least 80 minutes. In one embodiment of the method for preparing the modified lignin according to the present invention, the immersion time is in the range of from 30 minutes to 4 hours, preferably in the range of from 45 minutes to 3 hours, or even more preferably in the range of from 60 minutes to 2 hours.

[0051] In a preferred embodiment of the method for preparing the modified lignin according to the present invention, the acidic aqueous solution is stirred during the immersion step. Any suitable stirring means as known by a person skilled in the art may be used.

[0052] The modified lignin according to the present invention has reduced swelling tendency, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, in the range of from 0.8 to 3.0. The swelling tendency can be determined by measuring the volume of an approximately 1 gram dry lignin sample in a test tube at 20°C, then subjecting the lignin sample in the test tube to heating in an oven (ambient atmosphere) from 20°C to 250°C, at a predetermined temperature increase rate (heating rate), which in this case is 25°C / min. After the oven has reached 250 °C, it is maintained at this temperature for a predetermined hold time, in this case 51 minutes. The sample is then removed from the oven and allowed to cool to room temperature before measuring the volume of the lignin sample again. The swelling tendency, i.e. the ratio of the volume of the lignin sample after heating relative to the volume of the lignin sample before heating is then determined. Preferably, the swelling tendency, i.e. the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, is less than 2.0, even more preferably in the range of from 0.9 to 1 .5, such as from 0.9 to 1 .3 or 0.9 to 1 .2.

[0053] The modified lignin according to the present invention has a weight average molecular mass in the range of from 6000 g / mol to 15000 g / mol, such as 7800 g / mol to 15000 g / mol, preferably in the range of from 8000 g / mol to 10000 g / mol, such as from 8100 g / mol to 9200 g / mol or from 8200 g / mol to 9000 g / mol or from 8200 g / mol to 8800 g / mol. The weight average molecular mass can be determined as described in Jacobs A. et al., Nordic Pulp & Paper Research Journal (2000), vol 15, p 120-127.

[0054] The ash content of the modified lignin according to the present invention is preferably less than 0.1 wt-%, determined according to ISO 1762:2019.

[0055] The glass transition temperature of the modified lignin obtained in step c) is higher than the glass transition temperature of the unmodified lignin used as starting material in step a) of the method according to the present invention. It has been observed that the glass transition temperature increases by at least 10°C and typically increases by up to 30°C. Thus, the glass transition temperature of the modified lignin is 10° to 30°C higher than the glass transition temperature of the unmodified lignin. Preferably, the glass transition temperature increases by at least 10°C in the process according to the present invention, such that the glass transition temperature of the modified lignin is at least 12°C higher than the glass transition temperature of the unmodified lignin. Preferably, the glass transition temperature of the modified lignin is at least 147°C, such as at least 154°C, such as at least 158°C or at least 160°C or at least 162°C or at least 165°C or at least 168°C.

[0056] The particle size of the modified lignin obtained in step c) is increased compared to the particle size of the unmodified lignin used as starting material in step a) of the method according to the present invention. Particle size distribution can be determined using a Malvern Mastersizer 3000 in the same manner as described above for the untreated lignin. For such measurement, a lignin sample is sonicated for 120 seconds before the analysis. A refractive index of 1 .59 is used for calculating the particle size. It has been observed that the dv50, i.e. 50% by volume of the particles measured has a diameter smaller than this value, increases from about 5 to 20 pm to 30 to 150 pm in the method according to the present invention. In one embodiment, the dv50 for the modified lignin obtained in step c) is at least 50 pm, such as at least 70 pm or at least 80 pm. In one embodiment, the dv50 for the modified lignin obtained in step c) is in the range of from 60 to 140 pm, such as from 70 to 130 pm.

[0057] Step d) of the method for preparing the modified lignin according to the present invention involves separating the obtained modified lignin from the acidic aqueous solution. The modified lignin is in solid form during the separation. The term “separation” as used herein, refers to a process of separating the lignin from the acidic aqueous solution. Preferably, the separation in step d) is performed using filtration. Alternatively, the separation may be performed by centrifuging or sedimentation or any other suitable means known by a person skilled in the art. As the modified lignin is separated from the acidic aqueous solution, the pH of the acidic aqueous solution and the lignin during the separation step will be the same as the pH during the immersion step. Thus, the pH of the aqueous acidic solution is below 6, preferably below 5, or more preferably below 4, during the step of separating the modified lignin. In one embodiment, the pH of the aqueous acidic solution may be in the range of from 1 to 6, preferably from 2 to 6, more preferably from 2 to 5, and most preferably from 2 to 4.

[0058] Steps c-d and optionally e) may be repeated at least one time. The separated lignin is in this embodiment immersed in an acidic aqueous solution a second time, before a second separation step. The pH of the acidic aqueous solution and the temperature and time during the immersion step are selected as discussed above. If the immersion and separation steps are repeated, parameters such as pH, temperature and time during the immersion may be the same in all the immersion steps or they may vary between different immersion steps. In one embodiment, the same parameters are used during two immersion steps. In one embodiment, the pH may be lower in a second immersion step than in a first immersion step. In another embodiment, the immersion time may be shorter in a second immersion step than in a first immersion step. In yet another embodiment, the temperature may be higher in a first immersion step than in a second immersion step. The total immersion time is the sum of the immersion times for the individual steps. The separation is preferably performed by filtration after each immersion step.

[0059] Step e) of the method involves optionally washing the separated modified lignin. In one embodiment, the separated lignin is subjected to washing with an aqueous washing solution. The aqueous washing solution is preferably water. In one embodiment, the separated modified lignin is washed with water until the pH of the water used for washing becomes neutral.

[0060] In one embodiment, the method comprises an additional step of drying the separated, and optionally washed, modified lignin. The modified lignin may be dried after the separation step or after the optional washing step. The drying of the modified lignin may be carried out by methods and equipment known in the art. The temperature during the drying is preferably in the range of from 60°C to 160°C, more preferably in the range of from 100°C to 120°C. The drying may be performed under ambient pressure, reduced pressure or vacuum.

[0061] The modified lignin according to the present invention may have a total metal content in the range of from 0 to 200 ppm, such as from 0.1 to 200 ppm, preferably in the range of from 0 to 150 ppm, such as from 0.1 to 150 ppm and more preferably in the range of from 0 to 100 ppm, such as from 0.1 to 100 ppm.

[0062] Other inorganic impurities may also be removed from lignin to the acidic aqueous solution during the immersion step.

[0063] The modified lignin may be subjected to further treatments, such as various heat treatments.

[0064] It has been observed that the lignin becomes condensed during the immersion step. The glass transition temperature of the lignin increases and the tendency of the lignin to swell when heated is reduced. It has also been observed that the colour of the lignin becomes darker. The modified lignin comprises essentially only lignin, such as at least 99 wt% lignin based on the dry weight of the lignin material and less than 1 wt% of other components such as cellulose, hemicellulose and inorganic compounds, based on the dry weight of the lignin material. As mentioned above, the lignin is largely undissolved during the immersion step. Thus, the modified lignin obtained in step c) of the method for preparing the modified lignin according to the present invention is also in solid form, such as in the form of particles.

[0065] The modified lignin according to the present invention is particularly suited for further conversion to carbon enriched materials intended for energy storage applications as well as for other applications where a lignin material having iron content of less than 50 ppm, preferably less than 30 ppm or less than 20 ppm is of interest. The conversion to carbon enriched materials can be carried out according to methods known in the art, such as through WO202 1 / 250604.

[0066] The properties of the modified lignin according to the present invention means that it is particularly suitable for further conversion to a carbon enriched material that can be used in energy-storage applications.

[0067] Examples

[0068] In all examples, softwood Kraft lignin powder obtained from the LignoBoost process was used.

[0069] Example 1

[0070] Lignin was dispersed in water and concentrated sulfuric acid was added to the lignin based on the lignin weight. The following acid levels were used: 6, 9 and 18 (weight-% sulfuric acid of lignin dry weight). The lignin consistency was 10%. The lignin suspension was treated during 2 hours at 110°C in a glass vessel, the suspension was thereafter cooled to around 50°C and filtrated on a Buchner funnel. After filtration the lignin particles were washed with pure water on the funnel.

[0071] The swelling tendency of the lignin was measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating.

[0072] Particle size distribution of the lignin was be determined using a Malvern Mastersizer 3000. Each lignin sample was sonicated for 120 seconds before the analysis. A refractive index of 1 .59 was used for calculating the particle size.

[0073] The lignin was also analyzed using modulated differential scanning calorimetry. DSC (Differential Scanning Calorimetry) is a thermal analysis technique measuring heat flow in to and out of the sample as a function of time. MDSC is Modulated Differential Scanning Calorimetry. In MDSC a different heating profile is applied to sample and reference. The temperature of the sample changes continuously but not linearly. Because of the heating profile kinetic events such as melting, cross linking, crystallization and reversible heat flow which shows Tg (glass transition temperature) can be observed. The temperature is moderated ± 0.800 every 60 s and the heating rate is 5°C / min with heating until 230°C. Energy peaks can be evaluated. In the untreated kraft lignin an endothermic peak which shows melting can be observed. In the lignin treated according to the present invention a smaller endothermic peak or an exothermic peak can be observed, indicating crosslinking in the sample.

[0074] In Table 1 , the characteristics of the produced lignin is shown together with the starting material.

[0075] Table 1. Lignin characteristics.

[0076] The samples were also analyzed using scanning electron microscopy (SEM). A SEM instrument from Thermo Fischer was used. The working distance was 10 mm. Secondary electron mode was used with high vacuum and the beam used was around 10kV. Figure 1a shows an untreated lignin sample and Figure 1 b shows a lignin sample treated according to the present invention.

[0077] Example 2 - DoE study

[0078] A fractional factorial Design of Experiments (DoE) was undertaken to investigate the effect of various process parameters on the swelling and particle size (D50) of the modified lignins. Four independent variables were investigated, each variable having three levels as shown in Table 2.

[0079] Table 2. DoE independent variables and levels

[0080] Independent Minimum level Median level Maximum level variable

[0081] Lignin content 10 wt% 15 wt% 20 wt%

[0082] Acid content 0.25 wt% (pH 2.125 wt% 4 wt%

[0083] Reaction temperature 70 °C 86.5 °C 103 °C

[0084] Reaction time 30 min 75 min 120 min

[0085] In total, 29 experiments were conducted. The dependent variables were lignin swelling and lignin D50 particle size. Lignin swelling was determined in accordance with the method described above, however using a lower heating rate (10 °C / min) and shorter hold time (38 mins). A heating rate of 10 °C / min will typically give a somewhat lower swelling ratio as compared to the heating rate of 25 °C / min used to define the swelling tendency in the present application.

[0086] Swelling and particle size were each modelled as a function of the parameter space using multivariate linear regression performed with the MODDE DoE software package. Besides each independent variable, the product of each combination of two independent variables (including the squares of each independent variable) were also investigated for inclusion in the regression models in order to investigate interaction of variables.

[0087] Regarding decrease in swelling tendency, this was found to depend mainly on a combination of reaction temperature and reaction time. Figure 2 shows a plot of the determined coefficients (scaled and centered) for the decrease in swelling tendency relative to a reference lignin.

[0088] The predicted values for decrease in swelling (% decrease in swelling tendency relative to a reference lignin) showed good agreement with the experimentally determined values, as shown in Figure 3. The DoE models may be used to predict any process point within the modelled parameter space. For example, it is predicted that, when using a lignin content of 7 wt%, an acid content of 4 wt% and a reaction time of 60 minutes, a reaction temperature of 80 °C will still result in a lignin having poor swelling properties (swelling tendency of 3.7 with heating rate 10 °C / min). Increasing the reaction temperature to 90 °C whilst keeping the other parameters the same is predicted to result in a lower swelling tendency of 2.5.

[0089] Example 3 - Kilogram scale study

[0090] Modified lignin was prepared using approximately 1 kg of starting lignin. The reaction was performed in a 5L glass reaction vessel stirred at 450 rpm. The lignis was slurried in water and heated to 97 °C. Concentrated sulfuric acid was added to a final concentration of 4 wt% (relative to total slurry) to initiate the reaction. Samples were removed periodically from the reaction vessel and worked-up to provide lignin for analysis.

[0091] Figure 4 shows the swelling tendency for each of the obtained sample lignins, measured in the same manner as in Example 2 (heating rate 10 °C / min, hold time 38 mins). The time indicated for each sample is measured starting from when the reactor reached 97 °C. It can be seen that the decrease in swelling tendency proceeds relatively quickly, and the swelling tendency is decreased to less than 1 after 34 minutes of reaction.

[0092] Quantitative Phosphorous-31 NMR measurements of TMDP-derivatised lignin samples, performed using the method described in Granata et al. (Journal of Agriculture and Food Chemistry, 1995, 43, 1538-1544) but using encto-HNDI as internal standard (Holzforschung 2001 , 55, 283-285), show that the concentration of aliphatic hydroxy groups in the lignin decreases from about 1 .6 mmol / g in the untreated lignin to about 1 .2 mmol / g in the modified lignin with low swelling tendency.

[0093] The weight average molecular mass of the lignin increased from 5800 g / mol for the untreated lignin to 6500 g / mol for the modified lignin.

[0094] Determination of the surface and pore properties of the lignins by nitrogen adsorption and desorption using the BET method showed that the modified lignin had very low total surface area (0.17 m2 / g) and very small average pore size (7.98 A). In contrast, the untreated lignin starting material had a total surface area of 13 m2 / g and average pore size of 104 A.

[0095] Example 4 - Lignin-carbon composites

[0096] A carbon fines fraction (Dv50 approximately 3 pm) produced by the stabilization, carbonization and calcination of lignin followed by subsequent milling and sieving, was blended with lignin (Dv50 particle size of approximately 5 pm). Blends containing 5 wt%, 10 wt% and 20 wt% carbon fines were tested. The blend was suspended in water at 15 wt% dry substance content and the pH was adjusted to pH 0.5 with sulfuric acid. The suspension was heated to 110 °C in a reactor and stirred during 120 minutes. After the reaction, liquid was removed by vacuum filtration on a Buchner funnel, and the resulting lignin-carbon composite was washed and dried. Analysis of the composite showed that the Dv50 particle size of the treated material was approximately 24 pm, with no detectable particles below 10 pm in size. In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

Claims1 . Lignin having a swelling tendency, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, in the range of from 0.8 to 3.0.

2. The lignin according to claim 1 , wherein the swelling tendency of the lignin is in the range of from 0.9 to 1 .5.

3. The lignin according to claim 2, wherein the lignin has a glass transition of at least 147°C.

4. The lignin according to any one of claims 1 -3, wherein the average particle size, expressed as dv50and determined using using a Malvern Mastersizer 3000 of the lignin is at least 30 pm, such as at least 50 pm.

5. The lignin according to any one of claims 1 -4, wherein the lignin has a weight average molecular mass in the range of from 6000 to 10000 g / mol, such as 8000 to 10000 g / mol.

6. The lignin according to any one of claims 1 -5, wherein the lignin is Kraft lignin.

7. The lignin according claim 6, wherein the lignin is Kraft lignin from softwood.

8. The lignin according to any one of claims 1 -7, wherein the ash content thereof is less than 0.1 wt-%, determined according to ISO 1762:2019.

9. Carbon enriched material produced from the lignin according to any one of claims 1 -8.

10. Method for preparing modified lignin comprising the following steps: d) providing lignin in solid form; e) preparing a slurry comprising the lignin in an acidic aqueous solution, wherein the slurry has a pH below 1 .0, and treating the slurry at a temperature of from 90°C to 130°C for a duration t in order to obtain said modified lignin; and f) separating the obtained modified lignin from the acidic aqueous solution; wherein the lignin remains in solid form during all steps of the method.11 . The method according to claim 10, wherein the duration t is equal to or greater than the time required for the swelling tendency of the lignin, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, is in the range of from 0.8 to 3.0.

12. The method according to any one of claims 10-11 , wherein step b) consists of steps b1 ) and b2): b1 ) providing an acidic aqueous solution having a pH below 1.0; and b2) immersing the lignin in the acidic aqueous solution, wherein the temperature of the acidic aqueous solution during the immersion is in the range of from 90°C to 130°C, until the swelling tendency of the lignin, measured as the ratio of the volume of the lignin, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min,to the volume of the lignin before being subjected to the heating, is in the range of from 0.8 to 3.0, to obtain said modified lignin.

13. The method according to any one of claims 10-12, wherein the slurry has a lignin content of 10 wt% or greater relative to the total weight of the slurry.

14. The method according to any one of claims 10-13, wherein the pH is below 0.8.

15. The method according to any one of claims 10-14, wherein the duration t is in the range of from 30 minutes to 4 hours.

16. The method according to claim 15, wherein the duration t is in the range of from 45 minutes to 2 hours and the temperature is increased during the duration.

17. Method for preparing a lignin-carbon composite, the method comprising the following steps: a) providing lignin in solid form; b) providing a carbon powder; c) preparing a slurry comprising the lignin and the carbon powder in an acidic aqueous solution, wherein the slurry has a pH below 1 .0, and treating the slurry at a temperature of from 90°C to 130°C for a duration t in order to obtain said lignin-carbon composite; and d) separating the obtained lignin-carbon composite from the acidic aqueous solution; wherein the lignin remains in solid form during all steps of the method.

18. The method according to claim 17, wherein the carbon powder has a Dv50 particle size that is less than or equal to the Dv50 particle size of the lignin.

19. The method according to any one of claims 17-18, wherein the carbon powder is a fines fraction obtained in a process for the production of carbon from lignin.

20. The method according to any one of claims 17-19, wherein the ratio by weight of lignin to carbon powder in the slurry is from about 1 : 1 to about 99:1.21 . Lignin-carbon composite obtainable by the method of any one of claims 17-20.

22. Lignin-carbon composite consisting essentially of lignin and carbon, and having a swelling tendency, measured as the ratio of the volume of the lignin-carbon composite, in dry form, after being subjected to heating from 20°C to 250°C, at a temperature increase rate of 25°C / min, to the volume of the lignin before being subjected to the heating, in the range of from 0.8 to 3.

0.

23. Lignin-carbon composite according to any one of claims 21 -22, wherein the ratio by weight of lignin to carbon in the composite is from about 1 :1 to about 99:1.

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