METHOD FOR FRACTIONAL SEPARATION OF LIGNIN

Description

[0001] The present invention relates to a process for the fractional precipitation of lignin from a non-aqueous solution, wherein in a first process step a) the lignin is dissolved in a solvent selected from the group consisting of substituted or unsubstituted aliphatic, cyclic ethers or lactones or a mixture of these solvents and in a second process step b) the lignin is selectively precipitated from the solution by applying different carbon dioxide pressures at least as a function of the molecular weight and / or as a function of the lignin composition.

[0002] The industrial potential of biological materials has been exploited for many years in a wide variety of applications. For example, the manufacture of furniture or clothing using only natural products is familiar from everyday life. A further dimension has emerged in the industrial sector in recent decades, with the focus not only on the pure use of available materials but also on resource-efficient, specific processing and reprocessing. This combination is intended to specifically develop new properties and areas of application for the basic materials. For the most part, economically important biological materials are mixtures of polymeric substances, the individual fractions of which each contribute their own share to the macroscopic properties of the overall material. This basic principle is particularly evident in the use of wood.Wood is a mixture of different biopolymers, with the individual fractions, such as lignin and cellulose, each exhibiting a distinctly different structure and responsible for specific properties of the macroscopic wood system. This overarching structural principle also extends to individual biopolymers. For example, lignin, with a proportion of 20-40%, plays a central role in the biological processing of lignocellulose. This fraction can be used particularly as an innovative biomaterial or for the resource-efficient, non-petroleum-based synthesis of chemicals with aromatic components.The understanding of the chemical and physical properties of different lignin fractions is still in its infancy, but it is certain that only with the tailored industrial use of the specific advantages of individual lignin fractions in life cycle-driven processing steps can an improved economic balance be achieved.

[0003] The patent literature contains a wide variety of proposals for the processing of biopolymers, especially lignin.

[0004] For example, EP 3 030 599 B1 describes a process for isolating lignin, preferably unmodified lignin, from a mixture, in particular a slurry in pulp production, containing lignin, a solvent for cellulose, a precipitant for cellulose, minerals and optionally (still present) cellulose and / or hemicellulose and / or hydrolysis products thereof, comprising or consisting of the following steps: i) removing the precipitant for cellulose from the mixture, ii) bringing the mixture into contact with a precipitant for lignin, preferably at a temperature below 50°C, to obtain a precipitate, iii) removing the precipitate obtained in step ii) from the mixture, iv) optionally washing the precipitate with a precipitant for lignin, v) removing the precipitant for lignin from the precipitate,vi) optionally separating the solvent for cellulose and the precipitant for lignin from the remaining mixture obtained in step iii), with the proviso that step i) is only optional if the precipitant for cellulose is the same as the precipitant for lignin used in step ii) or comprises one or more components of the mixture used as precipitant for lignin in step ii).

[0005] In another patent document, US 8 172 981 B2, a process for separating lignin from a black liquor is disclosed, which comprises the following steps: a) precipitating lignin by acidifying black liquor, resulting in a first lignin suspension and then performing a separation, thereby forming a first cake of solid material, which is a first lignin filter cake, b) suspending the lignin filter cake obtained in step a) and, during the suspension process, adjusting the pH of the suspension to approximately the pH of the wash water from step c) below, whereupon a second lignin suspension is obtained; c) separating the second lignin suspension, thereby forming a second cake of solid material, i.e., a second filter cake, and then adding acidified wash water for displacement washing, followed by improving the dryness of the lignin filter cake, whereupon an acid filtrate is obtained; and d) conveying the filtrate from step c) or selected portions of the filtrate from step c) for dilution prior to separation in step a) and / or adding it as wash water to the filter cake produced in step a), the ionic strength being adjusted prior to addition by adding salt, ESP dust, pulp mill kettle dust, or sodium sulfate;to control a Na / S balance in a pulp mill, and the pH is adjusted by adding ESP dust or pulp mill kettle dust and the additional use of the filtrate in step d) takes place in the suspension step of step b) or before the filtration step of step c).

[0006] Furthermore, WO 0 238 857 A1 discloses a process for reducing the content of wood extractives or for substantially removing wood extractives, comprising extracting the inconal wood extractives and removing the extract from the fibers to obtain improved cellulose fibers with a reduced content of extractives.

[0007] Further information on lignin processing can be found, for example, in Smink Dion et al.: "Comparing multistage liquid-liquid extraction with cold water precipitation for improvement of lignin recovery from deep eutectic solvents", SEPARATION AND PURIFICATION TECHNOLOGY, Vol. 252, 1 December 2020 (2020-12-01), page 117395, XP55934038 and Myint Aye Aye et al.: "One pot synthesis of environmentally friendly lignin nanoparticles with compressed liquid carbon dioxide as an antisolvent", GREEN CHEMISTRY, Vol. 18, No. 7, 1 January 2016 (2016-01-01), pages 2129-2146, XP055806638. Such solutions known from the state of the art may offer further potential for improvement, particularly with regard to the specificity of the separation process and the flexibility in the lignins to be separated.

[0008] It is therefore the object of the present invention to at least partially overcome the disadvantages known from the prior art. In particular, the object of the present invention is to disclose a process by which lignins can be separated very precisely according to their molecular weight and / or structure. Furthermore, the object of the present invention is to disclose the use of this process for separating lignin.

[0009] The object is achieved by the features of the respective independent claims, directed to the method according to the invention, as well as the inventive use of the method.

[0010] Accordingly, the invention relates to a process for the fractional precipitation of lignin from a non-aqueous solution, wherein in a first process step a) the lignin is dissolved in a solvent selected from the group consisting of substituted or unsubstituted aliphatic, cyclic ethers or lactones or a mixture of these solvents and in a second process step b) the lignin is selectively precipitated from the solution by applying different carbon dioxide pressures at least as a function of the molecular weight and / or as a function of the lignin composition.

[0011] Surprisingly, it was discovered that, using the above-described method and the specified group of solvents, a wide variety of lignins from different biological sources can be separated very easily and with high specificity. Thus, very precisely defined lignin classes can be obtained using the specified separation method, which is extremely important for the industrial utilization of lignins. Based on the heterogeneity of the biological, polymeric material, the obtainment of defined fractions now also makes it possible for the first time to consider industrial applications that are not easily feasible with nonspecific lignin mixtures due to their chemically and physically heterogeneous properties.In this respect, a narrower classification of lignin substance classes can be provided for the first time. In addition to their specific properties, these classes also have the advantage that the material can be obtained from natural, renewable sources. Furthermore, the process can be made more environmentally friendly by using solvents from biological sources or by obtaining them using biological processes. Overall, this can have an extremely positive impact on the ecological balance of the entire process. Without being bound by theory, it is assumed that the use of a solvent from the group of small to medium-sized, O-heterocyclic aliphatics, in combination with the use of carbon dioxide as the fraction-determining agent, leads to particularly suitable changes in the solvent properties, which, in conjunction with the aromatic basic structures of the lignins, contribute to particularly efficient separation.A wide variety of different lignins can be dissolved in these solvents at moderate temperatures in high weight fractions, which can contribute to efficient process control. Furthermore, the solvent properties in this group of solvents can be precisely adjusted as a function of the applied carbon dioxide pressure, allowing the lignins to be precipitated from the solution with particularly high homogeneity as a function of their molecular weight and / or chemical structure. In this context, it is also surprising that the changes in solvent properties react particularly sensitively to the linkage of the individual lignin subunits, so that, in particular, a highly functional separation as a function of the linkage is also achieved in the process according to the invention.

[0012] The process according to the invention is a process for the fractional precipitation of lignins from a non-aqueous solution. The process according to the invention therefore not only provides the possibility of precipitating lignin from a solution. The process according to the invention enables specific lignin fractions to be precipitated successively from the solution by changing the carbon dioxide pressure. Therefore, it is not according to the invention that, in a single-step process, all of the lignin held in solution is precipitated by increasing the carbon dioxide pressure. In particular, fractional precipitation can involve, for example, precipitating higher-molecular-weight lignin fractions at lower carbon dioxide pressures and lower-molecular-weight lignin fractions at higher carbon dioxide pressures. The lignins are precipitated from a non-aqueous solvent.A non-aqueous solvent is present in particular when the water content of the solvent is less than or equal to 50 percent by weight. The water content of the solvent can preferably be less than or equal to 25 percent by weight, further preferably less than or equal to 5 percent by weight. In particular, the non-aqueous solvent can advantageously be anhydrous. In this context, anhydrous means that the water content in the solution comes largely from the lignins and not from the solvent. In particular, the water content after dissolving the lignin in the solvent can be less than or equal to 0.1 percent by weight.

[0013] Lignins form a group of phenolic macromolecules composed of different monomer building blocks, depending on their biological origin. Lignins belong to the class of biological polymers and are commonly found in trees and grasses. In these plants, lignins cause the lignification of cells. Approximately 20 to 30 percent of the dry mass of woody plants can be attributed to lignins, making them, along with cellulose and chitin, among the most abundant organic compounds on Earth.

[0014] All lignins belong to the class of phenylpropanoids, with molecular weights of approximately 5,000–10,000 Da typically being achieved for individual lignin polymers. Additional substituents on the benzene ring can be short aliphatic groups, such as propyl, hydroxy, and / or methoxy groups, as well as alkoxy or aryloxy groups. Lignin polymers form three-dimensional amorphous networks, with the individual lignin chains being linked together in a variety of ways. In addition to aromatic bonds, lignins contain other carbon-carbon single or double bonds, as well as phenolic groups. The exact composition of lignins depends on their plant origin, with p-coumaryl, coniferyl, and sinapyl alcohol occurring in varying proportions as the specific basic building blocks.

[0015] The process comprises a first process step a) in which lignins are dissolved in a solvent selected from the group consisting of substituted or unsubstituted aliphatic, cyclic ethers or lactones or a mixture of these solvents. In the first process step, the lignins are added to the solvent and dissolved therein. This can be accelerated, for example, by stirring, ultrasonic treatment, or heating the solvent. The dissolution of the lignins in the solvent does not occur on the basis of the splitting of the chains into the individual monomers, but rather by the separation of the individual polymer chains from one another. The dissolution of the lignins can, for example, take place at a temperature of greater than or equal to 10 °C and less than or equal to 60 °C.The proportion of lignin to be dissolved can vary depending on the solvent; for example, lignin contents of greater than or equal to 5 weight percent and less than or equal to 30 weight percent can be dissolved in the solvent. The oxygen-containing heterocycles mentioned above have proven particularly suitable for the appropriate separation of the individual lignin polymers. Without being bound by theory, the cyclic, non-aromatic ring structures of the solvents appear to be suitable for reducing the interactions between the individual lignin chains and contributing to a particularly rapid and complete dissociation and dissolution of the lignins. Cyclic ethers are understood to be closed aliphatic monocycles that have at least one oxygen atom in the ring structure.Ethers can also be polyethers, which can, for example, carry multiple oxygen atoms in the ring structure. Typical sizes of cyclic compounds can, for example, have 4–15 atoms in the ring structure. The same applies to lactones as solvents as for ethers, although lactones naturally also contain at least one additional oxygen atom adjacent to the ring oxygen (= O) and not incorporated into the ring structure. In cases where the solvents are substituted, the specified basic structures can carry additional substituents such as alkyl chains or other functional groups, such as hydroxyl groups, alkoxy groups, nitrogen-containing functional groups such as amino groups, etc. The molecular weight of the additional substituents does not exceed the molecular weight of the ring structure.

[0016] The process comprises a second process step b), in which the lignin is selectively precipitated from the solution by applying different carbon dioxide pressures, at least as a function of the molecular weight and / or as a function of the lignin composition. After dissolving the lignin in the organic solvent, the pressure can be gradually increased by injecting carbon dioxide. The carbon dioxide dissolves in the solvent and thereby changes the solvent properties of the solvent. By using the solvents listed above, the solvent properties of the solvent can be varied very precisely over a wide range, particularly by means of carbon dioxide.Without being bound by theory, the oxygen heterocycles usable according to the invention, in conjunction with the use of carbon dioxide, appear particularly suitable for the particularly selective removal of different lignin fractions from the solvent. The precipitated lignin fraction can be mechanically separated from the rest of the solution. Furthermore, the different lignin fractions can be advantageously precipitated in this solvent group using a relatively limited range of carbon dioxide pressures. For example, within the process according to the invention, pressures of greater than or equal to 5 bar and less than or equal to 100 bar can be used to separate the individual lignin fractions. The solvent properties in conjunction with the injection of carbon dioxide separate the lignin polymers not only as a function of molecular weight.It is also possible to separate the lignin based on its monomer composition or even on the individual monomer fractions. This results in a very flexible system that can also be used, for example, based on the applied pressure jumps, to separate polymers with the same molecular weight but different monomer compositions.

[0017] In a preferred embodiment of the process, the solvent of the following formula I or II or mixtures of these compounds, where R1m is present at any bondable site of the cyclic framework and is each independently selected from the group consisting of H or C1-C4-alkyl, with m = 3+n for formula I and m = 2+n for formula II, and n = 1, 2, 3, 4, 5. In particular, the group of ethers and lactones mentioned above has proven particularly suitable for separating a large number of different lignins. By applying different carbon dioxide pressures, the properties of the solvent can be varied particularly widely, so that even with low pressure ranges a broad range of molecular weights and / or polymer compositions can be separated. In these solvents the lignins dissolve very quickly and at moderate temperatures, so that complex tempering of the solution is unnecessary.Another advantage is that the solvents can be obtained from natural starting materials. This can increase the overall environmental compatibility of the process. Furthermore, these solvent groups can be easily and completely removed from the precipitated lignin, thus producing very pure lignin fractions without significant energy expenditure.

[0018] Within a further preferred embodiment of the process, the solvent of the following formula III or mixtures of these compounds, where R 1< m is present at any bondable position of the cyclic skeleton and is each independently selected from the group consisting of H or C1-C4 alkyl, where m = 4. In particular, the substituted or unsubstituted tetrahydrofurans can be suitable for separating lignins of different composition or molecular weight. The individual lignins can be obtained with a very small polydispersity index, which indicates a very specific fractionation of the lignins using these solvents. This group of solvents can also be obtained from biological sources, which can in particular improve the environmental balance of the proposed process. The indication of the index m indicates that the tetrahydrofuran can, for example, carry one, two or even four alkyl chains.By choosing the substitution pattern, the solvent can be further adapted to the separation task at hand, and in particular to the source of the lignin used.

[0019] In a further preferred aspect of the process, the solvent can be 2-methyltetrahydrofuran. In particular, the use of 2-methyltetrahydrofuran (2-MTHF) can contribute to the rapid and very precise fractionation of lignins. The solvent is very efficient, and thus, very selective molecular weight ranges of lignin fractions are obtained under only low carbon dioxide pressures. The polydispersity of the individual fractions is improved compared to the use of other solvents.

[0020] In a further preferred embodiment of the process, in addition to lignin, other biopolymers from the group consisting of lipids, saccharides, and proteins, or mixtures of at least two components from this group, can be separated. In addition to the processing of lignins, the presented process can also be used to process other biopolymers, for example, from the above-mentioned group. The chemical composition of these polymers is suitable for separation into more homogeneous fractions in the specified group of preferred solvents, using carbon dioxide, selectively as a function of molecular weight and / or as a function of the composition of the individual polymers.

[0021] Within a preferred aspect of the process, the lignin can have a weight-average molecular weight M w , determined by size exclusion chromatography (SEC), of greater than or equal to 100 g / mol and less than or equal to 10,000 g / mol. Particularly in the range of lignin fractions with a low molecular weight, the presented process can contribute to the precipitation of the individual polymers from the solvent in a very narrow size distribution. As a function of the applied carbon dioxide pressure, fractions can be obtained which differ only very slightly in their molecular weight and / or fractions which have an approximately identical composition of the monomers in the polymer chain. More preferably, the molecular weight can also be less than or equal to 6,000 g / mol. In particular, these ranges can be obtained from lignins which originate from an Organosolv or OrganoCat pretreatment.

[0022] Within a further preferred embodiment of the process, the lignin can contain a proportion of β- O -4 linkages of greater than or equal to 10 mol% and less than or equal to 60 mol%, based on the number of monomer units within the molecule. The process according to the invention can be used in particular to obtain different lignin fractions that differ in the type of linkages of the individual monomers. Lignin polymers that exhibit β-O-4 linkages in the range specified above are particularly suitable for this purpose. This class of lignins can arise, for example, from the type of lignin processing and can be processed in combination with the process according to the invention to produce particularly uniform fractions.

[0023] Within a further preferred aspect of the process, the lignin can originate from the chemical processing of a lignocellulose, wherein the processing of the lignocellulose to separate the lignin comprises at least contacting the lignocellulose with an organic solvent. The composition and physico-chemical form of the lignin are significantly influenced by two factors. As already stated above, the composition of the lignin varies greatly depending on the plant origin. In addition to the lignin source itself, the chemical processing of the original plant material can also result in significant differences in the appearance of the lignin fractions to be separated. The kraft process, which is used to produce cellulose, has long been technically established. In the kraft process, lignin arises as a by-product of the lignin / cellulose mixture used.The lignocellulose used is separated into lignin and cellulose in an alkaline aqueous solution with the addition of sodium sulfide, the latter being used, for example, in paper production. Due to the harsh process conditions and the use of sulfur, the lignin is only obtained in a contaminated state. Furthermore, the process conditions, particularly the high temperatures and the alkalinity of the solvent, result in lignin of very low quality, which in most cases can only be used for thermal recycling. The primary focus of this process is the production of high-quality cellulose. However, to obtain higher-quality lignin fractions, processing methods that completely dispense with alkaline steps and high temperatures have proven effective. These processes also use lignocellulose as a starting material.However, this is converted under moderate process conditions in an organic solvent (organo) and thus separated from the other components of lignocellulose. This lignin contains no sulfur components and can also be distinguished by a higher proportion of β-. O -4 linkages. Due in particular to the higher proportion of these linkages and the gentler treatment of the lignin, this material can be particularly well separated into further fractions using the process according to the invention. The fractions obtained from this processing are significantly more homogeneous, characterized by low polydispersity, and can exhibit significantly more uniform properties.

[0024] Furthermore, the invention relates to the use of the method according to the invention for the separation of lignin fractions obtained from an OrganoCat or Organosolv process. The method according to the invention can be used in particular in the context of a selective, environmentally friendly, and cost-effective lignin processing process. Lignin is advantageously obtained through a specific pretreatment process in which lignocellulose is separated into its three main components: hemicellulose, cellulose, and lignin. Pretreatment methods advantageous for lignin quality are carried out using organic solvents (Organosolv), which replace the harsh, water-based and acid-catalyzed (Kraft) processes. The improved reaction conditions trigger more selective depolymerization of the polysaccharide fraction(s).The use of organic solvents also physically separates the lignocellulose components, thus obtaining the various raw materials. OrganoCat pretreatment is specifically aimed at reducing lignin degradation during separation by using biogenic resources and relatively mild processing conditions. OrganoCat pretreatment also uses an organic solvent and an organic carboxylic acid as a catalyst (e.g., oxalic acid or 2,5-furandicarboxylic acid, FDCA). A biphasic medium consisting of water and 2-methyltetrahydrofuran (2-MTHF) can be used as the solvent, with the organic solvent being obtained from biogenic resources. Relatively mild conditions are used in the OrganoCat process (e.g.,140-160 °C, 1-3 h), so that the acid selectively depolymerizes the hemicelluloses to obtain xylose and other sugars, while cellulose and lignin are retained as macromolecules. The cellulose remains suspended in the aqueous phase, and the lignin is extracted using the 2-MTHF phase. The lignin can then be recovered from the 2-MTHF phase for further use. In addition to distillation, the process according to the invention is particularly suitable for further processing, since the lignins are already dissolved in the "right" solvent due to the process and can be separated purely by using carbon dioxide. Furthermore, the lignin structure differs significantly from the structure resulting from other pretreatments, such as kraft, which leads to improved separation using the process according to the invention.Overall, in addition to the advantages in process management, there is also an improved quality of the available, separated lignins.

[0025] Further advantages and advantageous embodiments of the objects according to the invention are illustrated by the examples and drawings and explained in the following description. Examples Pretreatment Lignocellulose - Lignin and Cellulose

[0026] For the fractional separation of lignin, beech lignocellulose is separated into lignin and cellulose using an OrganoCat process. The beech wood was cut to approximately 1 mm and dried. 0.1 M oxalic acid and 2-MTHF are added to the dried chips. The mixture is heated to 140°C for 3 h. The mixture is cooled to room temperature, and the two liquid phases are separated using a decanter. The cellulose collects in the aqueous phase. The lignin-containing organic phase is extracted using CaCl2 solution to remove the oxalic acid. Fractional precipitation of different lignins

[0027] The OrganoCat lignin is dissolved in 2-MTHF at a concentration of 10 wt.% and placed in a high-pressure reactor at room temperature. The solution is pressurized with carbon dioxide at different pressures, inducing lignin precipitation in different fractions. Equilibrium at each pressure level is reached after approximately 15 minutes. The precipitate is collected, dried, and analyzed for its chemical composition.

[0028] The precipitation of lignin from the organic 2-MTHF solution can be observed starting at a pressure of approximately 10 bar. Within a pressure range of up to 50 bar, a total of 70 wt.% of the lignin used is precipitated from the solution.

[0029] Figure 1 shows the weight fraction of precipitated lignin as a function of the applied carbon dioxide pressure. The figure shows both the weight fraction of precipitated lignin (bars) per stage and the integral amount of precipitated lignin (dotted line).

[0030] The Figure 2shows the proportions of the different lignin subunits as a function of the separated fractions, i.e. p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units (left axis). In addition, the proportion of units per 100 monomer units of the lignin is given (right axis). It can be seen that the lignin remaining in solution consists predominantly of H units (up to 16%), whereas virtually no H units are present in the precipitated lignins in the pressure range of 0-50 bar. From the results, it can also be concluded that the low-molecular components, which do not precipitate at all or precipitate at higher pressures, have higher H contents. The figure also shows that the S and G units have an equal proportion (approximately 40-55%) at the respective pressure levels. The latter is an indication that the proportions of these monomers are statistically distributed in all lignin fractions with higher molecular weight.

[0031] The Figure 2 shows both the distribution of relevant links (β-β, β- O -4, β-5) as well as the occurrence of monomeric, dimeric and oligomeric structures as a function of the applied carbon dioxide pressure. Lignin fractions with a higher β- O -4-component are precipitated at relatively low carbon dioxide pressures in the range of 10-20 bar. At higher pressures, the β- O -4 content is slightly lower. This demonstrates that the process presented here can be used to selectively provide fractional separation of lignins with different and higher β-O-4 contents. The fraction precipitated at 10 bar shows a β- O-4 fraction. The ratio of the other linkages, i.e. β-β and β-5, is independent of the carbon dioxide pressure and remains almost constant across the different fractions. Monomers and small oligomers are only slightly separated even with increasing pressure and collect in the final lignin fraction, which remains in the solution. The proportion of β-β and β- O -4 linkages in the still dissolved fraction suggests that these are relatively small oligomers with a high proportion of these linkages.

[0032] An analysis of the hydroxyl functions of the different lignin fractions reveals a constant monomer composition and a decreasing number of aliphatic hydroxyl groups, which precipitate with increasing pressure. A higher number of carboxylic acid groups can be detected in the lignin remaining in solution, which may be due to the oxalic acid used in the processing.

[0033] To further characterize the lignin fractions, the weight-average molecular weights and the polydispersity index (PDI) were determined using size-exclusion chromatography (SEC). With increasing carbon dioxide pressure, the molecular weight decreases, indicating lignin precipitation as a function of molecular weight. The available PDI of the individual fractions is lower than the PDI of the total fraction used. The fraction precipitated at 10 bar has a high proportion of β- O -4 linkages and S and G units with a weight-average molecular weight of approximately 4600 g / mol. The 50 bar fraction has a lower content of β- O-4 linkages with a molecular weight of approximately 1040 g / mol. The fraction remaining in solution at 50 bar pressure has a weight-average molecular weight of approximately 580 g / mol, which is approximately four times lower than the weight-average molecular weight of the lignin used. The latter fraction likely consists of lignin monomers and dimers.

[0034] By combining an OrganoCat process with the separation method according to the invention, it was demonstrated that it is possible to efficiently separate lignins using biogenic solvents with carbon dioxide. The pressure triggers a molecular weight-dependent precipitation of the lignin, whereby varying crosslinking fractions can be obtained in the different fractions. The solvent can be reused, thus providing a cost-effective and environmentally friendly route to obtaining specific lignins.

Claims

1. Process for the fractional precipitation of lignins originating from a chemical work-up of a lignocellulose from a non-aqueous solution, wherein the work-up of the lignocellulose for separation of the lignin comprises at least contacting the lignocellulose with an organic solvent, characterized in that, in a first process step a) lignin is dissolved in a solvent selected from the group consisting of substituted or unsubstituted aliphatic, cyclic ethers or lactones or a mixture of these solvents and, in a second process step b) the lignin is selectively precipitated from the solution at least as a function of the molecular weight and / or as a function of the lignin composition by application of different carbon dioxide pressures.

2. . The process according to claim 1, wherein the solvent is according to the following formula I or II or mixtures of these compounds, wherein the R1m are present at each bindable site of the cyclic skeleton and are each independently selected from the group consisting of H or C1-C4 alkyl, with m = 3+n for formula I and m = 2+n for formula II, and n = 1, 2, 3, 4, 5.

3. . The process according to any one of the preceding claims, wherein the solvent is of the following formula III: or mixtures of these compounds, wherein the R1m are present at each bindable site of the cyclic skeleton and are each independently selected from the group consisting of H or C1-C4 alkyl, wherein m = 4.

4. . The process according to any one of claims 2 or 3, wherein m = 1.

5. . The process according to any one of the preceding claims, wherein the solvent is 2-methyltetrahydrofuran.

6. . The process according to any one of the preceding claims, wherein the lignin has a weight average molecular weight Mw, determined by size exclusion chromatography SEC, of greater than or equal to 100 g / mol and less than or equal to 10,000 g / mol.

7. . The process according to any one of the preceding claims, wherein the lignin has a β-O-4 linkage content of greater than or equal to 10 mol% and less than or equal to 60 mol% based on the number of monomer units within the molecule.

8. . The process according to any one of the preceding claims, wherein the lignin is dissolved in the solvent at a weight fraction greater than or equal to 3% by weight and less than or equal to 25% by weight.

9. . Use of a process according to any one of the preceding claims for the separation of lignin fractions obtained from an OrganoCat or Organosolv process.