Lignin fractionation

Abstract

The present invention relates to a method for isolating a lignin fraction from a solution containing solubilized lignin, the method comprising the steps of reducing the concentration of the hydrotrope by adding water to a solution containing solubilized lignin and a hydrotrope until a portion of the lignin is insoluble, and isolating the insoluble lignin.

Description

[Technical field]

[0001] The present invention relates to lignin fractionation using hydrotropes. [Background technology]

[0002] Lignin is the second most abundant biopolymer on Earth after cellulose and is mostly obtained as a by-product of the paper and pulp industry and biorefineries.

[0003] The potential of lignin as a feedstock for high-value applications lies in its high availability, biodegradability and biocompatibility, as well as its antioxidant and antibacterial properties. The main drawback of using lignin as a feedstock is its heterogeneity and high polydispersity. Therefore, fractionation of lignin and recovery of polymer fractions with well-defined molecular weights (Mw) and low dispersity are crucial steps to appreciate lignin and develop new applications.

[0004] Possible applications of the various types of lignin include its use as fuel, the production of fine chemicals such as BTX (benzene, toluene and xylene) and derivatives, vanillin, phenols and organic acids. In addition, it can be used as the main or one of the components in cements, foams, resins, adhesives, dispersants and adsorbents. Furthermore, its interesting biological properties and biocompatibility give it potential use in biomedical applications such as drug carrier systems and tissue engineering.

[0005] Kraft lignin originates from the kraft process in the pulp industry and is currently the main source of lignin produced worldwide at several million tons per year. Most of this lignin is burned as fuel, but the high availability of this by-product and its interesting properties have prompted research developments in the isolation and purification of kraft lignin from black liquor.

[0006] LignoBoost Kraft lignin fractionation research interest is growing with several research groups around the world obtaining interesting results in fractionation and possible applications of this fractionated lignin.

[0007] So far two methods have been found in the literature for the fractionation of kraft lignin by organic solvent fractionation [Jiang et al. (2017) ACS Sustain. Chem. Eng. 5, 835-842] or membrane processes [Sevastyanova et al. (2014) J. Appl. Polym. Sci. (2014) 131, 9505-9515]. Organic solvent fractionation is based on the solubility of lignin fractions with different Mw in certain solvents such as ethanol, methanol and acetone. On the other hand, fractionation by membrane separation processes usually involves the fractionation of kraft lignin directly from black liquor or its recovery in a process different from Lignovoost using ultrafiltration membranes.

[0008] Hydrotropes are non-toxic, biodegradable, reusable organic salts that, when added in specific amounts, increase the solubility of certain solutes in water or other solvents. The main advantage of using hydrotropes is that the desired solute can be recovered whenever needed by diluting the solution with water to a specific concentration and reprecipitating the solute.

[0009] In recent years, lignin has been extracted from various types of softwoods and hardwoods by using hydrotropic solutions under specific conditions to separate the lignin from cellulose and hemicellulose. The lignin is then recovered by dilution with water, which causes the lignin to precipitate, and the hydrotropic solution can be reused several times before it loses its functionality. The hydrotropes most used in this specialized process are sodium xylene sulfonate, p-toluenesulfonic acid and maleic acid.

[0010] US3490990 discloses that lignin can be precipitated from a solution containing a hydrotrope by adding water. Wang et al. (2020) Indust. Crops Prod. 150, 112423 disclose that lignin can be precipitated by diluting p-TsOH to a concentration of less than 11.5%. Chen et al. (2017) Sci Adv 3 e 1701735) and US10239905 disclose that lignin precipitates from woody poplar biomass from about 16% p-TsOH, whereby further dilution to 4% increases the amount of precipitated lignin. Summary of the Invention

[0011] Hydrotropic extraction of lignin shows that by using different reaction temperatures, hydrotrope concentrations and reaction times it is possible to obtain different lignin fractions with different Mw and properties.

[0012] Generally, LignoBoost Kraft lignin (LKL) is insoluble in water but soluble in hydrotropic solutions. The present invention discloses the use of aqueous hydrotropic solutions to precipitate lignin fractions with specific Mw, lower dispersibility, and different contents of functional groups based on the solubility of the lignin fractions in different hydrotropic concentrations. The preferred hydrotropes selected for this process are sodium xylene sulfonate (SXS) and sodium cumene sulfonate (SCS). Moreover, the final hydrotropic solution can be reconcentrated by evaporation and reused several times.

[0013] The present invention shows that it is possible to fractionate LignoBoost Kraft lignin (LKL) by diluting hydrotropic solutions of Sodium Xylene Sulfonate (SXS) and Sodium Cumenosulfonate (SCS) with water. Selective precipitation of 100 g / L lignin into SXS solution occurs at hydrotrope concentrations up to 16 wt% and four different fractions can be recovered at hydrotrope concentrations of 16, 14, 12 and 10 wt%. The molecular weights (Mw) of the different fractions are between 24 kDa and 7 kDa and the visible polydispersity values ​​are decreasing for all fractions as confirmed by GPC.

[0014] Fractionation with SCS solution gave three distinct fractions at hydrotropic concentrations of 10, 8, and 6 wt%. The Mw of these fractions ranged from 24 kDa to 19 kDa. Fractionation of LKL with NaOH solution was not possible.

[0015] Characterization of the fractions and the original LKL sample by FTIR-ATR and H-NMR allowed us to analyze the overall structure of LKL and the fractions obtained. Furthermore, these two characterization methods allowed us to study the efficiency of the washing process to remove residual hydrotropes from the fractions.

[0016] The amount of water required to completely wash the fraction from the hydrotrope is about 200 parts by weight water to 1 part by weight dry lignin, but the hydrotrope can be recovered by evaporation and the wash water can be reused along with the final hydrotropic solution.

[0017] Furthermore, characterization by quantitative C-NMR results shows that the different fractions have different contents of phenolic and aliphatic OH and β-O-4' bonds. Fractions with lower hydrotropic % (lower Mw) have higher contents of phenolic OH and lower contents of aliphatic OH. Elemental analysis results confirm that there is no significant difference in sulfur content between the original Lignovoost sample and the different fractions, indicating that it is possible to remove residual hydrotropes from the fractions.

[0018] Mass balances indicate that the 16 wt% SXS and 8 wt% SCS fractions had the highest yields of lignin recovered from the initial solution, with approximately 72% and 78% of the dissolved lignin recovered in these fractions, respectively. Approximately 10-13% of the original lignin remained dissolved in the final hydrotropic solution.

[0019] The present invention is further summarized in the following description: 1. A method for isolating a lignin fraction from a solution containing solubilized lignin, the method comprising the steps of: a) reducing the hydrotrope concentration by adding water to a solution containing solubilized lignin and a hydrotrope until a portion of the lignin is insoluble, and b) isolating the insoluble lignin. 2. The method according to claim 1, wherein steps a) and b) are repeated, for example up to 5 times; 3. The method according to claim 1 or 2, wherein the solubilized lignin is kraft lignin. 4. The method according to any one of claims 1 to 3, wherein the lignin concentration of the solution containing solubilized lignin and hydrotrope is between 25 or 50 and 100 or 120 g / liter. 5. The method according to any one of claims 1 to 4, wherein the concentration of the hydrotrope in the solution containing solubilized lignin and hydrotrope is 30 wt%. 6. The method of any one of claims 1 to 5, wherein the hydrotrope is SXS or SCS. 7. The method of any one of claims 1 to 6, wherein the hydrotrope is SXS and the SXS concentration is reduced stepwise or continuously to 10 wt%. 8. The method of any one of claims 1 to 6, wherein the hydrotrope is SCS and the SCS concentration is reduced stepwise or continuously to 6 wt%, typically reduced stepwise. 9. The method of any one of claims 1 to 8, wherein the initial solubilized lignin and hydrotrope containing solution is diluted between 3 and 5 times with water. 10. Use of the method according to any one of claims 1 to 9 to enrich a lignin fraction enriched in aliphatic OH content, phenolic OH content or molecular weight range from a crude fraction of lignin. 11. A crude fraction of lignin or a lignin fraction enriched by the method according to any one of claims 1 to 9. [Brief description of the drawings]

[0020] [Figure 1] Diagram of the experimental procedure. [Diagram 2] Diagram of the experimental procedure [Diagram 3] FTIR-ATR spectrum of the original dried lignin sample. [Figure 4] FTIR-ATR spectra of lignin fractions recovered from selective precipitation using sodium xylene sulfonate (SXS). [Diagram 5] FTIR spectra of lignin fractions recovered from selective precipitation using sodium cumene sulfonate (SCS). [Figure 6] FTIR-ATR spectral comparison between the lignin fraction obtained by selective precipitation (16 wt%) and the original lignin. [Figure 7] FTIR-ATR spectral comparison between the lignin fraction obtained by selective precipitation (16 wt%) after additional washing and the original lignin. [Figure 8] H-NMR spectra of pure lignovoost lignin, pure SCS and pure SXS. [Figure 9]H-NMR spectra of pure lignin, the washed 16 wt% SXS fraction and the same fraction with residual hydrotropes. [Figure 10] C-NMR spectra of lignin-provided non-acetylated and acetylated softwood Lignovoost. [Figure 11] Molecular weight distribution of SXS fractions. [Figure 12] Molecular weight distribution for SCS fractions. [Figure 13] FTIR-ATR spectral comparison between the lignin fraction obtained by selective precipitation (10 wt%) and the original lignin before and after additional washing. [Figure 14] FTIR-ATR spectral comparison between the lignin fraction obtained by selective precipitation (12 wt%) and the original lignin before and after additional washing. [Figure 15] FTIR-ATR spectral comparison between the lignin fraction obtained by selective precipitation (14 wt%) and the original lignin before and after additional washing. [Figure 16] FTIR-ATR spectra of pure hydrotropic powders, SXS and SCS. [Figure 17] C-NMR spectrum of an acetylated sample from the SXS fraction. [Figure 18] C-NMR spectrum of a non-acetylated sample of the SXS fraction. [Figure 19] C-NMR spectrum of an acetylated sample of the SCS fraction. [Figure 20] C-NMR spectrum of a non-acetylated sample of the SCS fraction. [Figure 21] C-NMR spectra of pure SCS and pure SXS.

[0021] Detailed Description Lignin is a three-dimensional, heterogeneous polymer composed mainly of three monomeric units known as p-hydroxyphenyl, guaiacyl and syringyl units. Lignins vary, for example, with respect to molecular weight, phenolic OH content and aliphatic OH content. The lignin typically used refers to kraft lignin, which is used as a starting product in the process of the present invention and is a pure lignin composition.

[0022] More typically, the methods used in the present invention are carried out on compositions containing greater than 90 wt% lignin, greater than 93 wt% lignin or greater than 95 wt% lignin.

[0023] Such compositions contain small amounts of cellulose, hemicellulose or ash, typically less than 2.5 wt% cellulose or less than 1 wt% cellulose, less than 10 wt% hemicellulose, less than 5 wt% hemicellulose or less than 2.5 wt% hemicellulose and less than 2.5% ash, less than 1 wt% ash or less than 0.5 wt% ash.

[0024] Compositions having any combination of the above values ​​for lignin, cellulose, hemicellulose and ash are expressly disclosed herein.

[0025] Particular compositions for use as feedstock in the methods of the invention include greater than 93 wt% lignin, less than 1 wt% cellulose, less than 5 wt% hemicellulose and less than 1 wt% ash.

[0026] Kraft lignin is known in the art and is lignin obtained from the Kraft process, in which lignocellulosic material is treated with sodium hydroxide and sodium sulfide. Lignin is also recovered from black liquor. A more specific type of lignin is obtained via the Lignoboost™ process, which uses CO2 and H2SO4 as a method to precipitate and purify lignin from black liquor. This lignin has a low sulfur content (between 1 and 3 wt%) and an ash content between 0.3 and 1.2 wt%.

[0027] "Water" as used in the process of the present invention refers to solutions such as drinking water, river water, groundwater or industrial wastewater from industrial processes. The water may be the wash water of the process of the present invention and may, in certain embodiments, contain small amounts of hydrotropes (less than 2 wt%).

[0028] The examples of the present invention carried out dissolution of the lignin at room temperature, i.e., between 20 and 25° C. Thus, the method of the present invention can be carried out without heating or adding heated water. Thus, depending on the area in which the lignin according to the present invention is to be treated, steps a) and b) can be carried out at temperatures ranging from 10, 20, or 25 degrees up to 30, 35, or 40° C. All ranges with any of the above lower and upper limits are contemplated and expressly disclosed herein.

[0029] The present invention shows that it is possible to fractionate lignin preparations such as softwood lignoboosted kraft lignin (LKL) using hydrotropic solutions of hydrotropes such as SXS and SCS. By diluting with water to reduce the hydrotrope concentration and the solubility of the lignin, various fractions are obtained. The insoluble lignin fraction is isolated by centrifugation and then washed to remove residual hydrotropes.

[0030] The stepwise dilution allows for fractionation of lignins of different chemical composition.

[0031] Characterization of the original lignin and the different fractions obtained by FTIR-ATR and H-NMR determined the amount of water required to remove all hydrotropic residues from each fraction.

[0032] Characterization by GPC shows that the subsequent stepwise precipitation leads to fractions with different Mw and polydispersity: the lower the hydrotropic concentration, the lower the polydispersity.

[0033] Characterization by quantitative C-NMR showed that the different fractions have different chemical group contents, with the lower Mw fractions having higher contents of phenolic OH groups and lower contents of aliphatic OH groups. Lignin fractions with higher contents of phenolic groups are usually accompanied by more reactive lignin fractions, making these lignins suitable for further chemical or physical modification for future applications. Elemental analysis shows no significant differences between the C, H and N contents of the original lignin and the fractions fractionated by SXS and SCS. The sulfur content also does not differ significantly between the raw material and fractionated lignin, illustrating that the fractionation method of the present invention removes organically bound sulfur.

[0034] Mass balances show that 16 wt% SXS and 8 wt% SCS are the highest yielding fractions, and also the fractions with the highest Mw of 24 kDa and 21 kDa, respectively. The molecular weight of the 16 wt% SXS fraction is almost the same as that of the original lignin, but the polydispersity index is significantly reduced from 10.4 to 7.6. The 10 wt% SXS fraction yields only about 1% lignin, but is the lowest molecular weight fraction with about 7 kDa.

[0035] Additionally, hydrotropes can be recovered from the wash water and reused in future fractions, reducing the amount of water required to complete the different fractions. EXAMPLES

[0036] Example 1. Dissolution of Kraft Lignin in Hydrotropic Solutions. The behavior of Lignoboost Kraft lignin in hydrotropic solutions was examined by dissolving freeze-dried lignin samples in aqueous solutions of sodium xylene sulfonate (SXS) and sodium cumene sulfonate (SCS).

[0037] For the hydrotropic solutions, the following procedure was followed: 1. Duplicate aqueous solutions of both SXS and SCS with hydrotrope concentrations of 10, 20, 30, and 40 wt% were prepared by mixing the hydrotrope with deionized water at room temperature. 2. Freeze-dried lignin was added to each solution at a concentration of 25 g / L. An additional control solution of deionized water only was included. 3. One solution was stirred at 25°C and the other solution was placed in a mixing water bath at 70°C for 15 minutes. 4. After the dissolution experiment, the solutions were analyzed for the presence of undissolved lignin.

[0038] Solubility limits of kraft lignin in hydrotropic solutions. After dissolving lignin in hydrotropic solution at 25 g / L, the same experiment was repeated with increasing concentrations of lignin to verify the solubility limit of lignin in the solution. A 30 wt% hydrotrope solution was prepared for the hydrotropic solution. Lignin concentrations of 35 g / L, 50 g / L, 100 g / L, 150 g / L, 200 g / L and 250 g / L were tested for both SXS and SCS hydrotropic solutions. 2 wt% NaOH solution was used for lignin concentrations of 50 g / L and 100 g / L.

[0039] Selective precipitation of lignin in hydrotropic solutions. After determining the solubility limit of lignin in both solution types, six solutions with lignin concentration of 100 g / L and hydrotrope concentration of 30 wt% were prepared and diluted with water to hydrotrope concentrations of 20, 10, and 5 wt% to examine the possibility of lignin reprecipitation in both hydrotropic solutions.

[0040] Fractionation of kraft lignin for valorization. After confirming the possibility of precipitation of lignin dissolved in the hydrotropic solutions, experiments were carried out to verify in what range of hydrotropic concentrations the lignin fraction could be obtained. For both hydrotropes, a fresh 30 wt% hydrotropic solution was prepared using 100 g / L and diluted in 2 wt% increments from 20 to 6 wt% based on the previous concentration by addition of deionized water. The precipitate was separated from the remaining hydrotropic solution by centrifugation. This procedure was carried out in two different ways. Firstly, by carrying out a direct dilution from the original 30 wt% hydrotropic concentration to each of the desired concentrations, such as from 30 wt% to 16 wt%, or from 30 wt% to 10 wt%.

[0041] Furthermore, fractionation was performed using a method termed "dilution series," in which the original 30 wt% hydrotropic solution was diluted to 16 wt% and the precipitate was collected, for example by centrifugation, and the supernatant with the dissolved lignin was then used for the next dilution, allowing the recovery of a less polydisperse fraction.

[0042] Advanced characterization of lignin fractions The recollected fractions were freeze-dried for 48 hours to remove the water that had collected in the precipitate. For advanced characterization of the fractions, various techniques were employed.

[0043] FTIR-ATR characterization To evaluate and compare all chemical structures, the collected fractions, the original Lignovoost Kraft lignin and the pure hydrotrope were analyzed by FTIR-ATR. The instrument used was a Bruker Alpha with 24 scans and a resolution of 4 cm. -1 It was.

[0044] NMR characterization H-NMR and quantitative C-NMR were performed on all fractions collected from SXS and SCS, as well as the original lignin sample. For H-NMR analysis, approximately 80 mg of lignin was dissolved in 0.55 mL of deuterated DMSO and spectra were recorded on a Bruker Avance Ill HD 400 at 25 °C.

[0045] For quantitative C-NMR, a sample preparation procedure based on the literature was used [Balakshin et al. (2015) RSC Advances 5, 87187-87199]. Lignin acetylation was performed on all lignin fractions and on the original kraft lignin. For this, a total of 4 mL of a 1:1 (v / v%) mixture of anhydrous pyridine / acetic anhydride was mixed with 150–200 mg of dry lignin. The solution was stirred at room temperature for 24 h. Afterwards, pyridine and traces of acetic anhydride were removed with ethanol. 10 mL of ethanol was added and the solution was stirred for another 30 min. The ethanol was evaporated and the procedure was repeated 7–8 times until the lignin was completely clean and dry.

[0046] Finally, the samples were washed with water and freeze-dried.

[0047] For the C-NMR procedure, 200 mg of acetylated or non-acetylated lignin was mixed with 0.50 mL of deuterated DMSO, 0.06 mL of the relaxation solution chromium(III) acetylacetonate (0.016 M), and the internal standard (IS) trioxane (IS:lignin ratio 1:10, w / w). The final solution was transferred to an NMR tube. A total of 16 samples were analyzed by C-NMR. Quantitative C-NMR spectra were recorded on a Bruker Avance 600 MHz at 25.0 °C. Inverse gate detection and 90 0A pulse width of 1.1 s was used. 20,000 scans were collected with an acquisition time of 1.1 s and a relaxation delay of 2.0 s. The spectrum was Fourier transformed, phased, calibrated, and the baseline was manually corrected using a polynomial function. The baseline was adjusted to zero using the following approximate interval ranges: (220-215 ppm)-(185-182 ppm)-(97-94 ppm)-(5-(-20) ppm). No other ranges were forced to zero. The aromatic region of the spectrum (approximately 100-163 ppm) was integrated and calibrated to a value of 600. Subsequent integrations of regions of interest in this spectrum are done in units of "per 100 Ar".

[0048] The amount of specific groups in mmol / g of lignin was calculated based on the following equation:

[0049]

number

[0050] where X: Amount of specific part I x、 I IS and I AC correspond to the resonance values ​​of a specific site, an internal standard, and all acetylated groups (corresponding to all OH), respectively. m lig and m IS is the weight of lignin and internal standard. Furthermore, 30 is the equivalent mass of IS (M: 90 g / mol) with 3 equivalent carbons resonating at approximately 92 ppm, and 42 is the increment in mass of lignin after acetylation of each OH group.

[0051] Gel Permeation Chromatography (Chromatoqraphv) GPC analysis was performed by dissolving the dried lignin samples in DMSO / LiBr (0.5% w / v) and shaking overnight. Prior to GPC analysis, the solutions were filtered through 0.45 μm PTFE syringe filters. The equipment used included an autosampler, a column oven, a UV detector equipped with a Dionex HPLC Pump Series P580 (Dionex Softron GmbH, Germering, Germany), a Dawn HELEOS MALS detector equipped with a 785 nm laser and a refractive index detector. The MALS detector was equipped with a narrow bandpass filter. Separation was performed using an Agilent PolarGeI M guard column (7.5V50mm) and three PolarGeI M columns 7.5V300mm (5mm particle size).

[0052] elemental analysis For elemental analysis of CHNS, 1–2 mg of dried original lignin and lignin fractions were weighed into tin capsules along with vanadium (V) oxide for sulfur analysis. The equipment used was a Flash 2000 elemental analyser.

[0053] Technical analysis of fractionation For mass balance, the recovered lignin fraction was washed with water and the fraction was analyzed by FTIR-ATR and H-NMR to determine the required water content and ensure that there were no residual hydrotropes in the fraction.

[0054] The fractions were freeze-dried for 48 h to remove all water and the recovered lignin yield in each fraction was calculated by the following equation (Equation 3):

[0055]

number

[0056] For the wash water, the following procedure was used: 1. The hydrotropic fraction obtained by centrifugation was washed with deionized water. The fraction was stirred to ensure that any dissolved hydrotrope in the solid fraction was dissolved in the wash water. 2. The new solution containing the collected fractions and wash water was centrifuged again to separate the wash water from the fractions. 3. The wash water from each fraction in which the hydrotrope had dissolved was saved for hydrotrope recovery.

[0057] To determine the amount of hydrotrope recovered in the wash water, the wash water was collected separately for each fraction, and then a certain amount of the wash water was completely evaporated to obtain the final amount of hydrotrope in the wash water.

[0058] The wash water was finally concentrated to 30 wt% of the original hydrotrope.

[0059] The final hydrotropic supernatant, 10 wt % in SXS and 6 wt % in SCS, was concentrated to 30 wt % of the original by evaporation and mixed with concentrated wash water to obtain a hydrotropic solution with the same initial volume as the starting solution.

[0060] Example 2 Dissolution of Kraft Lignin in Hydrotropic Solution When 25 g / L lignin was dissolved in hydrotropic solutions of different concentrations, it was found that for hydrotropic concentrations of 10 wt% and 20 wt% (for both hydrotropes), it was not possible to completely dissolve all the lignin in both experiments at 25 °C and 70 °C. This was only achievable for hydrotropic concentrations of 30 wt% and 40 wt%. In the water-only control samples, no lignin was dissolved.

[0061] Although the final results were similar at 25 °C and 70 °C in terms of the amount of undissolved lignin at the bottom of the test tube, the dissolution process was faster in the 70 °C experiment, allowing the total dissolution of the lignin to be reached sooner. This phenomenon was observed by the change in the color of the solution. The solution prepared at 25 °C was light brown for several hours until all the lignin was dissolved and the solution turned dark brown, while the solution prepared at 70 °C quickly became dark brown after a 15 min heating procedure. Since the solubility limit of the hydrotrope solution in water is about 40 wt%, this concentration was used as the maximum concentration to avoid precipitation of the hydrotrope. Furthermore, when the concentration of lignin was 25 g / L, all the lignin was dissolved in the 30 wt% hydrotropic solution, so this hydrotropic concentration was chosen for the next experiment. After determining the conditions for the hydrotropic solution experiment described below, the solubility limit of lignin in the 30 wt% hydrotropic solution was tested. The results show that in both hydrotropes a maximum concentration of lignin can reach 150 g / L, however, because these solutions are very viscous, a concentration of 100 g / L was used in subsequent experiments.

[0062] Example 3 Selective precipitation of lignin in hydrotropic solution To check the possibility of diluting the hydrotropic solutions to lower concentrations to recover lignin fractions, deionized water was added to 30 wt% hydrotropic solutions containing 100 g / L lignin concentration to 20, 10 and 5 wt%. At 20 wt% there was no precipitation in either solution, which was unexpected since the original dissolution experiments showed that the lignin was undissolved at this hydrotrope concentration. The 10 wt% aqueous sodium xylene sulfonate solution showed precipitation with a yellow clear supernatant. At 5 wt%, both hydrotropic solutions also produced a similar supernatant to the 10 wt% one. Another set of experiments was performed to understand whether it was possible to obtain different lignin fractions between 20 wt% and 5 wt% SXS solutions and between 10 wt% and 5 wt% SCS solutions.

[0063] Example 4. Fractionation of Kraft Lignin for Valorization Starting from a 20 wt% SXS solution, different dilutions were carried out at room temperature to verify the possibility of obtaining different fractions. The results of these experiments are shown in Table 1.

[0064] [Table 1]

[0065] There is no precipitation in the 18 wt% SXS fraction, but by diluting the 20 wt% solution to a lower concentration of hydrotrope, it is possible to recover the lignin fraction, which is separated from the residual liquid by centrifugation.

[0066] In hydrotropic fractionation experiments with SCS, precipitation was only possible at weight concentrations of 10, 8, and 6 wt%.

[0067] Example 5. Advanced characterization of lignin fractions a) FTIR-ATR analysis For advanced characterization of the lignin fractions and the original lignin sample, the FTIR spectra obtained are shown in Figures 3 to 7.

[0068] The original lignin spectral peaks were identified by comparison with similar spectra found in the literature [Abdelaziz et al. (2017) Waste and Biomass Valorization 8, 859869; Chen et al. (2016) RSC Advances 6, 107970-107976], and the identification of these peaks is shown in Table 2.

[0069] [Table 2]

[0070] To understand how the chemical structure of the recovered lignin fractions was changed by hydrotropic fractionation, these fractions were also freeze-dried and analyzed by FTIR.

[0071] Figure 4 shows that the spectra obtained for all fractions are similar, indicating that all the different fractions collected from fractionation by SXS do not have significant changes in their chemical structure. Figure 5 shows the chemical structures of the fractions obtained from fractionation by SCS.

[0072] The first fraction contained residual hydrotrope, as seen in the example of the 16 wt% SXS fraction in Figure 6. The amount of water required to remove the residual hydrotrope was analyzed first by FTIR and then by HNMR.

[0073] The spectra of both the lignin fraction and the original lignin show similarities throughout the FTIR spectrum (Figure 6), which was not the case initially when the fractions were not further washed after precipitation. Certain peaks identified in Figure 6 showed significant differences. To demonstrate the possibility of residual hydrotropes remaining in the precipitation, both hydrotropes were analyzed by FTIR and the different peaks were compared to the spectrum of the original hydrotrope. After several washes, the lignin fraction was freeze-dried again and analyzed by FTIR. The results of the additional washes are shown in Figure 7.

[0074] The FTIR spectrum of the 16 wt% rewashed fraction was virtually identical to that of the original lignin sample, indicating that the fraction had been cleaned from residual hydrotropes and presented an identical chemical structure to pure lignovoost lignin.

[0075] b)NMR analysis Regarding H-NMR analysis, the most important information obtained from this analysis for lignin samples is the purity of the fractions after washing to remove residual hydrotropes. FTIR can quickly and directly indicate the presence of residual hydrotropes, but H-NMR using a 400 MHz instrument is a more sensitive technique. In H-NMR, the lignin samples are not acetylated.

[0076] To understand the differences between the spectrum of the initial pure lignin and the spectra of the samples obtained by fractionation, the spectra of the different fractions were compared with the H-NMR spectra of the original lignin sample and the pure hydrotropes. Figure 8 shows the spectra of the pure lignovost lignin as well as the pure hydrotropes used for fractionation (SXS and SCS).

[0077] H-NMR peak identification was based on the literature [Amadou et al. (2015) BioResources 10, 4933-4946; Shiming & Lundquist. (2007) Nordic Pulp & 10 Paper Research Journal 9, 191-195].

[0078] The spectra in Figure 9 show the difference between the spectrum of the washed 16 wt% SXS fraction and the spectrum of the 16 wt% SXS fraction containing hydrotrope residue, and although fractionation does not cause any significant changes to the lignin structure of the fraction, when the fraction is not thoroughly washed the only difference seen in the spectrum shows peaks associated with the hydrotrope residue.

[0079] Considering the quantitative C-NMR results, calculations of mmol / g lignin for the different sites were made based on the literature (Balakshin and Capanema (2015), supra). Quantification of most chemical groups is based on values ​​obtained from both the acetylated and non-acetylated spectra of the samples. The table below (Table 3) contains the quantification of the most important sites of the original Lignovoost lignin and the areas of the spectrum where these sites are defined. The particular type of Lignovoost sample used is a softwood that does not contain any S units and has a low proportion of H units.

[0080] [Table 3]

[0081] Quantitative C-NMR is one of the most reliable characterization techniques for lignin, and characterization of kraft lignin always adds an additional challenge due to changes in the lignin structure during the kraft process. The main objective of this characterization method when studying lignin fractions is to evaluate the amount of aliphatic OH groups, phenolic OH groups, and carboxyl groups, which are related to lignin reactivity. Based on the literature (Wang, Luyao et al. (2020) ACS Sustainable Chemistry & Engineering 8(35), 13517-13526), ​​it is expected that lignin fractions with lower Mw will have higher contents of phenolic OH groups and carboxyl groups and lower contents of aliphatic OH groups. In addition, lignin fractions with lower Mw are expected to have lower contents of β-O-4' bonds, which are the most common bonds between lignin monomer units. According to the literature, higher phenolic content is associated with more reactive lignin, which may be advantageous for future applications and chemical modifications.

[0082] Figure 10 shows the C-NMR spectra of the acetylated and non-acetylated pure Lignovoost samples, along with the identification of some regions of interest in the spectra. The spectra of the non-acetylated and acetylated lignin fractions obtained in the hydrotropic fractionation are shown in the Appendix, along with a total of 16 C-NMR spectra collected.

[0083] As shown in Figure 10, acetylation of the lignin sample allows the separation of the total OH groups into two distinct regions of the spectrum, which makes it possible to quantify this portion in a more specific manner than in the spectrum of non-acetylated lignin.

[0084] Tables 4 and 5 show the phenolic and aliphatic contents and β-O-4′ linkage quantification of the different recovered lignin fractions, respectively.

[0085] [Table 4]

[0086] [Table 5]

[0087] The different obtained fractions have different amounts of aliphatic and phenolic OH as well as β-O-4' bonds, as shown in Tables 4 and 5. The trend of higher phenolic OH content and lower aliphatic OH content with decreasing Mw is evidenced in the section provided on GPC results.

[0088] c) Gel Permeation Chromatography (Chromatoqraphv) (GPC) The Mw and polydispersity of the different fractions and the original lignovost lignin were measured by a specific GPC technique. Technical lignins, especially kraft lignin samples, are known to fluoresce after UV absorption, which can affect the results obtained in standard GPC systems, resulting in higher or lower Mw instead of the correct value. In this analysis, a system equipped with a fluorescence filter on the MALS detector connected to the GPC instrument was used to avoid the interference of Mw due to this phenomenon. From this technique it is possible to obtain several parameters that describe the molecular weight distribution of the polymer, such as the number average molecular weight (Mn), the highest peak molecular weight (MP), the high order average molecular weight (Mz), the weight average molecular weight (Mw) and the polydispersity index (Mw / Mn).

[0089] Tables 6 and 7 show the GPC analysis of the fractions obtained by fractionating kraft lignin using SXS and SCS, respectively, and Figures 11 and 12 show the molecular weight distribution graphs of the fractions obtained using both hydrotropes.

[0090] [Table 6]

[0091] [Table 7]

[0092] The examples demonstrate that fractionation of lignovoost lignin with hydrotropes SXS and SCS is possible, resulting in fractions with lower Mw at lower hydrotrope concentrations. Furthermore, polydispersity decreases with decreasing hydrotrope concentration. Comparing the GPC results with the C-NMR results, the lower Mw fractions show higher phenolic OH content and lower aliphatic OH content, as expected.

[0093] d) Intrinsic Viscosity and Molecular Weight The relative molecular weights of the fractions were determined by an indirect method that relates their MW to the intrinsic viscosity of the solution. The Mark-Houwink-Sakurada constants used in the calculations were 0.4165 and 0.23 for K and a, respectively. The lignin fractions were dissolved in 2 wt% NaOH aqueous solution and the viscosity was measured at 30°C. The results are shown in Tables 8 and 9.

[0094] [Table 8]

[0095] [Table 9]

[0096] Although this is not an absolute method to measure Mw, the results of Mw of the fractions are not exact molecular weights, it still allows to see that the relative molecular weights of the fractions are different and thus confirm the hypothesis of fractionation of kraft lignoboosted lignin by selective precipitation with hydrotropic solutions. Moreover, this method can be used in laboratories or pilot plants where GPC equipment is not available to know the Mw of the fractions during the process.

[0097] elemental analysis The results of the elemental analysis are shown in Tables 10 and 11.

[0098] [Table 10]

[0099] Table 10 shows the elemental analysis of the pure lignoboosted lignin and the fractions obtained by fractionation with SXS. The results show that there is no significant difference between the original sample and the SXS fractions for C, H and N. Regarding the sulfur content, only the 16 wt% SXS fraction shows a higher content than the original lignin. However, this additional amount of sulfur cannot come from residual hydrotropes in the sample, since a preliminary NMR analysis was performed to ensure that there were no residual hydrotropes in the sample before the elemental analysis. The lower sulfur content in the 14, 12 and 10 wt% SXS fractions than in the original lignin could be due to the extensive washing of the fractions to remove elemental and inorganic sulfur, leaving only organically bound sulfur.

[0100] [Table 11]

[0101] The data in Table 11 show that there are no significant differences between the original lignin and the fractions in terms of C, H and N content, however the 10 wt% and 8 wt% SCS fractions have higher sulfur content than the original lignin, indicating that these samples have higher sulfur content than the lower Mw 6 wt% fraction.

[0102] Example 6. Engineering Analysis of Fractions Mass Balance All collected fractions were washed and freeze-dried for mass balance determination. The yield of each fraction was then calculated based on the initial amount of lignin in the initial hydrotropic solution. Tables 12 and 13 show the yields of the different fractions obtained from the SXS and SCS experiments.

[0103] [Table 12]

[0104] [Table 13]

[0105] Between 10 and 13% of the original lignin remains dissolved in the final supernatant. It is possible to recover this remaining dissolved lignin, but the hydrotropic concentration must be reduced to a value of 2 wt%, since there is no precipitation of the fraction below 10 wt% SXS and 6 wt% SCS. The amount of water required to reach a hydrotrope of 2 wt% does not compensate for the recovered lignin. After recovery of the final fraction, both in SXS and SCS, the dissolved lignin can be recovered using filters, membranes or centrifugation, and the remaining hydrotropic solution can be concentrated to 30 wt% and reused in a new fractionation cycle.

[0106] The ratio of water per gram of dry lignin required to completely remove the residual hydrotropes from the different lignin fractions is about 200 parts by weight of water to 1 part by weight of dry lignin, which is a significant amount of water, but this water can be reused.

[0107] After the centrifugation step, the concentrations of hydrotropes remaining in the wet lignin fraction (sediment) and the supernatant for each fraction were similar, meaning that the hydrotropes present in the fractions were removed with the water and were retained in the wash water.

[0108] To reuse the hydrotrope dissolved in the wash water for future fractions, the wash water solution can be evaporated to concentrate it to an initial hydrotrope concentration of 30 wt%, and the concentrate can be reused for future dilution and washing steps. Thus, although a significant amount of water is required for the initial fraction, recovering and reusing this water can make the process water more efficient and reduce the costs associated with this utility.

[0109] Additionally, each precipitated fraction after washing contains approximately 7.5% solid lignin. The remainder of each fraction is water, which is currently removed by freeze-drying.

[0110] Solvent Recovery and Reuse Recovery and reuse of the solvents is still under investigation: recovery of the final hydrotropic solutions corresponding to the supernatants of the fractions of 10 wt% for SXS and 6 wt% for SCS is already possible, but the exact number of times these solutions can be reused to produce the same fractions without losing their hydrotropic power is still under investigation.

[0111] Additionally, water recovered from the evaporation step required to concentrate the diluted supernatant and wash water to the original hydrotropic concentration of 30 wt% can be reused for future dilution and wash steps, and this utility can reduce future costs and provide a more environmentally friendly process.

[0112] Example 7 FTIR-ATR Characterization For FTIR-ATR characterization, all the spectra of the different obtained fractions were compared with the original lignin. The figure below shows a comparison of the spectra of the original lignin and the fractions before and after the additional washing.

[0113] The FTIR-ATR spectra of both hydrotropes used in the fractionation are shown in FIG.

[0114] Example 8 Additional quantitative C-NMR spectra The following figures, Figures 17 to 20, show the C-NMR spectra of acetylated and non-acetylated samples of different fractions obtained by fractionating kraft lignin using SXS and SCS. As explained above, the difference in the spectra of the acetylated and non-acetylated samples is that the OH groups in lignin are changed by the acetyl groups and appear in different regions of the C-NMR spectrum, making the quantification of aliphatic and phenolic OH groups easier.

[0115] The spectra of the fractions obtained from fractionation by SXS and SCS are almost identical to that of pure lignin in terms of chemical structure, the main differences are only seen when performing peak integrations and the associated calculations of sites in mmol / g lignin. All peak integrations and calculations were performed using TopSpin software.

[0116] Additionally, C-NMR spectra of pure SXS and pure SCS were recorded to understand whether there was any hydrotrope impurity in the spectra of the collected fractions, and these spectra are shown in Figure 21.

Claims

1. A method for obtaining a lignin fraction rich in aliphatic OH content, phenolic OH content or molecular weight range from a solution containing solubilized lignin, the method comprising the following steps: a) a step of reducing the hydrotrope concentration by adding water to a solution containing solubilized lignin and a hydrotrope until a part of the lignin becomes insoluble, and b) a step of isolating the insoluble lignin, The method of repeating steps a) and b) one or more times and further reducing the hydrotrope concentration by adding water.

2. A method in which the solubilized lignin contains more than 93 wt% lignin.

3. The method according to claim 1 or 2, wherein the solubilized lignin is kraft lignin.

4. The method according to claim 1 or 2, wherein step a) is carried out at a temperature below 40°C.

5. The method according to claim 1 or 2, wherein the solution containing solubilized lignin and a hydrotrope has a lignin concentration between 25 or 50 and 100 or 120 g / litre.

6. The method according to claim 1 or 2, wherein the solution containing solubilized lignin and a hydrotrope has a hydrotrope concentration of 30 wt%.

7. The method according to claim 1 or 2, wherein the hydrotrope is SXS or SCS.

8. The method according to claim 1 or 2, wherein the hydrotrope is SXS and the SXS concentration is reduced stepwise or continuously to 10 wt% SXS.

9. The method according to claim 1 or 2, wherein the hydrotrope is SCS and the SCS concentration is reduced stepwise or continuously to 6 wt% SCS, typically stepwise.

10. The method according to claim 1 or 2, wherein the first solution containing solubilized lignin and a hydrotrope is diluted between 3 and 5 times with water.

11. Use of the method according to claim 1 or 2 for increasing the lignin fraction rich in aliphatic OH content, phenolic OH content or molecular weight range from a crude lignin fraction.

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