A method for preparing high-polymerization-degree, high-purity hemicellulose

Abstract

The present invention provides a method for selectively extracting high-polymerization degree hemicellulose by mild treatment of molten salt hydrate, comprising the following steps: (1) weighing an appropriate amount of bleached pulp raw material and immersing it in a molten salt hydrate at a temperature of 25-55°C for 0.5-7h; the pulp raw material in the molten salt hydrate has a slurry concentration of 1-10wt%, and the molten salt hydrate is Li + Mg 2+ , Ca 2+ 、Zn 2+ 、Al 3+ 、Fe 3+ and Cu 2+ (2) after impregnation, solid-liquid separation is performed, and an appropriate amount of regeneration solvent is added to the liquid phase to clean and desalt the precipitate to obtain hemicellulose, while the solid phase is directly cleaned and desalted to obtain high-purity cellulose; the amount of the regeneration solvent used is 1-1.5 times the mass of the molten salt hydrate. The preparation method not only achieves the co-production of high-polymerization hemicellulose and high-purity cellulose, but is also environmentally friendly and low-cost, solves technical problems in actual production, and has broad market prospects.

Description

Technical Field

[0001] The invention belongs to the field of polymer materials, and particularly relates to a method for preparing high-polymerization-degree and high-purity hemicellulose by treating with molten salt hydrate. Background Art

[0002] Hemicellulose is a component of the primary and secondary plant walls, accounting for approximately one-third of the cell wall biomass. Compared to cellulose's high degree of polymerization and linear, uniform glucan structure, hemicellulose is a heteropolysaccharide with a low degree of polymerization (average degree of polymerization of approximately 200) and branched chains. Examples include xyloglucan, xylan, mannan, glucomannan, and β-(1→3,1→4)-glucan, all of which have a typical β-(1→4)-linked carbohydrate backbone structure. Due to its wide availability, low cost, and good biocompatibility, hemicellulose can not only be used to prepare bio-based materials but can also be converted into oligosaccharides, chemicals, fuels, and thermal energy. In addition, the abundant oxygen-containing groups on hemicellulose, such as hydroxyl, acetyl, and carboxyl groups, can be modified into hemicellulose derivatives through etherification, esterification, grafting copolymerization, etc., giving them new functions (Biotechnol. Adv., 2012, 30: 879; Prog. Polym. Sci., 2023, 140: 101675).

[0003] In plant cell walls, hemicellulose is primarily connected to cellulose and lignin through hydrogen and chemical bonds. Therefore, hemicellulose extraction is typically achieved by disrupting these bonds. Currently, alkali, acid, organic solvent, ionic liquid, deep eutectic solvent (DES), and hydrothermal extraction methods are widely studied (Bioresour. Technol., 2009, 100:3906). Because different extraction methods have varying degrees of effect on hemicellulose, the resulting morphology and structure vary significantly (Biomass Convers. Bior., 2020, 12:2501). For example, while acid, alkaline, and hydrothermal extraction methods are relatively efficient, they all lead to hemicellulose degradation, resulting in lower hemicellulose yields and a lower degree of polymerization. Materials prepared from hemicellulose with a low degree of polymerization have lower strength, making them unsuitable for subsequent applications. Furthermore, acid treatment is accompanied by side reactions, resulting in not only low hemicellulose purity but also damage to the reaction vessel caused by the corrosive acid solution. Although the conditions of organic solvent, ionic liquid and DES extraction methods are relatively mild, solvent recovery and purification are more difficult, which not only results in low purity of the obtained hemicellulose, but also high cost (Bioresour.Technol., 2021, 341:125757; Ind.Crop.Prod., 2015, 76:688).

[0004] In summary, in order to realize the industrial application of hemicellulose, on the one hand, it is necessary to obtain high-purity and high-polymer hemicellulose, and on the other hand, it is also necessary to reduce costs and be more environmentally friendly. At present, no relevant reports have been seen. Summary of the Invention

[0005] To address the current challenges of efficiently and cleanly preparing high-DP (>200) and high-purity hemicellulose in existing technologies, the present invention provides a method for selectively extracting high-DP hemicellulose using mild treatment with molten salt hydrate. This method not only avoids degradation during hemicellulose separation, achieving the co-production of high-DP (>200) hemicellulose and high-purity cellulose, but also allows for 100% recycling of the molten salt hydrate, making it environmentally friendly and low-cost. This method addresses technical challenges encountered in actual production and holds broad market potential.

[0006] The technical solution of the present invention is a method for preparing high-polymerization-degree and high-purity hemicellulose, comprising the following steps:

[0007] (1) Weigh an appropriate amount of bleached pulp raw material and immerse it in a molten salt hydrate at a temperature of 25-55°C for 0.5-7h; the pulp raw material has a slurry concentration of 1-10wt% in the molten salt hydrate, and the molten salt hydrate is Li + Mg 2+ , Ca 2+ 、Zn 2 + 、Al 3+ 、Fe 3+ and Cu 2+ chloride, bromide, iodate, perchlorate, nitrate or thiocyanate; the mass fraction of inorganic salt in the molten salt hydrate system is 40-70wt%; the bleached pulp raw material is hardwood pulp, softwood pulp, straw pulp, hemp pulp, bamboo pulp or bagasse pulp.

[0008] The present invention innovatively proposes a method for gently extracting hemicellulose using molten salt hydrate. Because the hydrogen bonds between hemicellulose and cellulose molecules and the hydrogen bonds within hemicellulose molecules are broken by the metal cations in the molten salt hydrate (the metal cations can coordinate with the oxygen on the hydroxyl groups of hemicellulose, thereby breaking the hydrogen bonds), hemicellulose can be dissolved in the molten salt hydrate. At the same time, by controlling the impregnation temperature, impregnation time and the amount of pulp raw material, the present invention achieves the goal of obtaining high-purity hemicellulose with a high degree of polymerization without destroying the hemicellulose structure. Unlike traditional organic ionic liquids, molten salt hydrate is a concentrated inorganic salt solution with a molar ratio of water to salt close to the maximum water and coordination number of its metal cations. In ideal MSHs, the metal cations are coordinated with water molecules, but the anions are free to move. In addition, the molten salt hydrate solvent has a low viscosity (fluidity similar to that of water) and is more adaptable to changes in water content. Traditional organic ionic liquids or DES have strict control over water content requirements and, to ensure the application effect, even do not allow water to be contained.

[0009] (2) After the impregnation is completed, the solid and liquid are separated, and an appropriate amount of regeneration solvent is added to the liquid phase to wash and desalt the precipitate to obtain hemicellulose, and the solid phase is directly washed and desalted to obtain high-purity cellulose; the amount of the regeneration solvent used is 1-1.5 times the mass of the molten salt hydrate. The regeneration solvent is one of water, ethanol, acetone and methanol.

[0010] In this step, the hemicellulose dissolved in the liquid is regenerated using a regeneration solvent, resulting in a highly degraded, high-purity hemicellulose with a degree of polymerization (DP) >200, close to the native DP of native hemicellulose in plant cell walls. Furthermore, the resulting cellulose pulp is highly pure, meeting the requirements for producing lyocell fibers. This demonstrates that the method described in this invention achieves the production of two high-quality industrial materials in a single reaction, thereby realizing the high-value utilization of commercial pulp.

[0011] Preferably, the preparation method further comprises the recovery of molten salt hydrate and regeneration solvent, specifically: separating the precipitate in the liquid phase in step (2), subjecting the remaining liquid phase to distillation separation to obtain a regeneration solvent and a salt solution, and then concentrating the salt solution to an initial concentration (the mass fraction of inorganic salts in the molten salt hydrate is 40-70wt%), which can be reused in step (1). Through the recovery step, the molten salt hydrate and regeneration solvent in the present application are recycled multiple times with almost zero loss; compared with the prior art, the preparation cost is greatly reduced. Therefore, the preparation method described in the present application is green, efficient and sustainable, and is of great significance for the high-value utilization of biomass.

[0012] Preferably, the molten salt hydrate is one or more hydrates of zinc chloride, calcium chloride, lithium bromide, ferric chloride or lithium perchlorate.

[0013] The hemicellulose obtained by the above-mentioned method is separated and has a purity of 100% and a high degree of polymerization (DP) of 200-230. It is well known to those skilled in the art that hemicellulose with a high DP has a wider range of applications in the field of hemicellulose materials and does not produce waste materials and waste liquids. Therefore, the hemicellulose prepared in this application has a wide range of applications, is more environmentally friendly, meets the requirements of green and efficient production, and has important industrial application prospects.

[0014] As mentioned above, the hemicellulose is high-purity hemicellulose and can be directly used to prepare high-purity xylose or directly utilized as a material.

[0015] The high-purity cellulose separated by the above method has a polymerization degree of 500-1000, an α-cellulose content of 92-98%, a hemicellulose content of 0-3%, an Fe content of 0-5ppm, and an ash content of 0.1-0.3%.

[0016] As mentioned above, the high-purity cellulose is suitable for preparing lyocell fibers.

[0017] Beneficial effects of the present invention:

[0018] (1) The present invention provides a new one-step method for preparing high-polymerization-degree high-purity hemicellulose and co-producing high-purity cellulose. The preparation method is not only simple in process and mild in conditions, but also has high product yield and good quality.

[0019] (2) The hemicellulose prepared by the method of the present invention has a yield of up to 65-95%, a degree of polymerization of up to 200-230, and a purity of 100%, which solves the problems of low polymer content, low purity, and low yield in the preparation of hemicellulose in the prior art; and the co-produced high-purity cellulose has a degree of polymerization of up to 500-1000, and an α-cellulose content of up to 92-98%, which can be dissolved in NMMO solvent, meeting the production requirements of lyocell fiber, and has broad industrial application prospects and high economic value.

[0020] (3) The molten salt hydrate and regeneration solvent used in the preparation method of the present invention are recycled and reused after solid-liquid separation, distillation and concentration, so no waste is generated in the reaction. Compared with the existing technology, not only the production cost is greatly reduced, but the process is also more green and clean. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Attachment Figure 1 This is a scanning electron micrograph of the hemicellulose separated in Example 1 after freeze-drying.

[0022] Attachment Figure 2 It is the lyocell fiber obtained by spinning high-purity cellulose in Example 1.

[0023] Attachment Figure 3This is a gel permeation chromatogram of the hemicellulose separated in Example 2.

[0024] Attachment Figure 4 This is the liquid phase spectrum of the sugar solution after the high-purity cellulose prepared in Example 2 is hydrolyzed into monosaccharides.

[0025] Attachment Figure 5 This is a transmission electron micrograph of the aqueous hemicellulose dispersion separated in Example 2.

[0026] Attachment Figure 6 It is the lyocell fiber obtained by spinning high-purity cellulose in Example 2.

[0027] Attachment Figure 7 is a gel permeation chromatogram of the hemicellulose separated in Example 3;

[0028] Attachment Figure 8 This is the NMR image of the hemicellulose separated in Example 3 dissolved in lithium bromide trihydrate. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to the embodiments.

[0030] In order to further understand the present invention, the embodiments of the present invention will be described in further detail below with reference to examples, but the embodiments of the present invention are not limited thereto, unless otherwise specified.

[0031] Example 1: Preparation of high-polymerization and high-purity hemicellulose by one-step method using ZnCl2 / CaCl2 molten salt

[0032] Step (1): Weigh 20 g of bleached hardwood pulp, soak the pulp in 10 times the volume of deionized water overnight, mechanically stir at 200 rpm for 20 minutes to loosen the pulp, control excess water, and freeze-dry the pulp for later use.

[0033] Step (2): Weigh 130 g ZnCl2 and 10 g CaCl2, add them to 90 g deionized water, and mix them evenly with magnetic stirring in a 25°C water bath and stabilize for 30 minutes. The corresponding inorganic salt mass fraction is 61%.

[0034] Step (3): Weigh 9.2 g of the bone-dry pulp prepared in step (1), add it to the molten salt hydrate prepared in step (2), and stir in a water bath at 25° C. for 7 h. The corresponding pulp concentration is 4%.

[0035] Step (4): Pour the mixture obtained in step (3) into a vacuum filtration device for solid-liquid separation, filter using a polypropylene filter membrane, and collect the filtered solid and liquid separately.

[0036] Step (5): The solid collected in step (4) is washed and desalted to obtain high-purity cellulose.

[0037] Step (6): The liquid collected in step (4) is added to 230 g of methanol solution to precipitate hemicellulose. The solid obtained after filtering and washing the mixed liquid is the separated hemicellulose, and the hemicellulose is freeze-dried to obtain hemicellulose powder.

[0038] (1) Determination of hemicellulose yield and degree of polymerization:

[0039] The hemicellulose yield is calculated based on the ratio of the mass of the separated hemicellulose powder to the mass of the hemicellulose in the raw material, according to the following formula:

[0040]

[0041] Gel permeation chromatography (GPC) was used to measure the number average molecular weight (Mn) of hemicellulose. The test conditions were a column temperature of 35°C, a sodium nitrate-disodium hydrogen phosphate solution (0.1M:0.001M) as the mobile phase, a flow rate of 0.50 mL / min, and approximately 2 mg of sample was dissolved in 1.50 mL of the mobile phase. 10 μL of sample was injected each time for detection. The degree of polymerization of hemicellulose was then calculated according to formula (2):

[0042]

[0043] In the above formula, Mn is the number average molecular weight of hemicellulose, and 150.129 is the molecular weight of the hemicellulose sugar ring.

[0044] (2) Hemicellulose purity determination:

[0045] The main chemical components of hemicellulose powder were analyzed according to NREL's standardized method. Specifically, a 300 mg sample was hydrolyzed with 72% sulfuric acid at 30°C for 1 hour, then diluted to a 4% sulfuric acid concentration with distilled water. The sample was then autoclaved at 121°C and hydrolysis continued for 1 hour. The supernatant was then analyzed by liquid chromatography. Based on the liquid chromatogram, hemicellulose (xylan, etc.) and impurities have different elution times, and the hemicellulose purity can be calculated based on the peak area (Formula 3).

[0046] Hemicellulose purity (%) = (c × 0.88 × V) / Xb × 100 (3)

[0047] In the above formula, c is the xylose concentration in the liquid (mg / mL), 0.88 is the conversion coefficient of xylose to xylan, V is the volume of the hydrolysis solution (mL), and Xb is the mass of xylan in the sample before pretreatment (mg).

[0048] (3) Determination of hemicellulose content in high-purity cellulose:

[0049] The components of the high-purity cellulose were analyzed according to the NREL method. The hemicellulose content in the high-purity cellulose was calculated from the peak area (Formula 4).

[0050] Hemicellulose content in high-purity cellulose (%) = (c × 0.88 × V) / Xb × 100 (4)

[0051] In the above formula, c is the xylose concentration in the liquid (mg / mL), 0.88 is the conversion coefficient of xylose to xylan, V is the volume of the hydrolysis solution (mL), and Xb is the mass of xylan in the sample before pretreatment (mg).

[0052] (4) Determination of α-cellulose content in high-purity cellulose:

[0053] Weigh 2g of the prepared, bone-dry cellulose sample into a 100-150mL dry beaker. Add 30mL of 175g / L NaOH solution to immerse the sample. Add the alkali solution according to the following procedure: First, add approximately 15mL of alkali solution and carefully stir with a flat-ended glass rod for 2-3 minutes to form a uniform paste. Then, add the remaining alkali solution and stir evenly and carefully for 1 minute, avoiding vigorous stirring. Cover the beaker with a glass watch glass and place it in a constant-temperature water bath at (20±0.5)°C for mercerization. After 45 minutes (including the alkali immersion time), add 30mL of distilled water at (20±0.5)°C to the beaker and stir carefully for 1-2 minutes. Then, transfer the slurry from the beaker to a constant-weight glass filter, spreading it evenly throughout the filter. Then, slowly filter the slurry using a vacuum pump. Repeat the filtration process 2-3 times until the fiber slurry is completely trapped. Then, under weak vacuum filtration, wash three times with 25 mL (20 ± 0.5°C) of 95 g / L NaOH solution. Once all the washing liquid has been filtered out, wash the filter with 400 mL of deionized water at 18-20°C. Then, add 400 mL of 2 mol / L acetic acid solution at 18-20°C to the filter until the α-cellulose is completely submerged. Immerse for 5 minutes, then continue washing with water until the washing liquid no longer reacts with acid. Remove the filter, transfer it to an oven, and dry it at (105 ± 2)°C to constant weight. The increased mass of the filter is the mass of the α-cellulose.

[0054] The α-cellulose content X1 (%) of bleached wood pulp is calculated according to formula (5):

[0055]

[0056] Where m1 is the mass of the glass filter after drying, g;

[0057] m2——mass of the glass filter containing dried α-cellulose, g;

[0058] m——mass of the absolute dry sample, g;

[0059] The α-cellulose content X2 (%) of bleached straw pulp is calculated according to formula (6):

[0060]

[0061] Where m3 is the mass of the glass filter after burning, g;

[0062] m4——mass of glass filter and ash after burning, g;

[0063] m1, m2, m are the same as above.

[0064] (5) Determination of degree of polymerization, ash content and Fe content of high-purity cellulose:

[0065] The degree of polymerization of high-purity cellulose was determined by the cuprammonia solution method.

[0066] The ash content in high-purity cellulose is determined according to GB / T2677.3-1993.

[0067] The Fe content in high-purity cellulose is determined according to GB / T 8943.2-2008.

[0068] According to the analysis results, in Example 1, the yield of hemicellulose is 70%, the purity of hemicellulose is 100%, and the degree of polymerization is 200; the degree of polymerization of high-purity cellulose is 850, the ash content is 0.13%, the Fe content is 0.2 ppm, the α-cellulose content is 92%, and the hemicellulose content in the high-purity cellulose is 0.3%. Figure 1 This is a scanning electron micrograph of the freeze-dried hemicellulose separated in Example 1.

[0069] In addition, the cellulose pulp (high-purity cellulose) obtained in step (5) was dissolved in N-methylmorpholine oxide (NMMO) solvent, and lyocell fibers were obtained by wet spinning. Figure 2 shown.

[0070] Example 2: Preparation of high-polymerization and high-purity hemicellulose by one-step method using ZnCl2 molten salt

[0071] Step (1): Weigh 25 g of bleached softwood pulp, place the pulp in a grinder to loosen it into a cotton-like state, and dry the loosened pulp until it is absolutely dry for later use.

[0072] Step (2): Weigh 130 g of ZnCl2, add it to 195 g of deionized water, and mix it evenly with magnetic stirring in a 25°C water bath and stabilize it for 30 minutes. The corresponding inorganic salt concentration is 40%.

[0073] Step (3): Weigh 22.75 g of the absolute dry pulp prepared in step (1), add it to the molten salt hydrate solution in step (2), and stir in a 45°C water bath for 0.5 h. The corresponding pulp concentration is 7%.

[0074] Step (4): Pour the mixture obtained in step (3) into a vacuum filtration device for solid-liquid separation, filter using a polypropylene filter membrane, and collect the filtered solid and liquid separately.

[0075] Step (5): The solid collected in step (4) is washed and desalted to obtain high-purity cellulose.

[0076] Step (6): The liquid collected in step (4) is added to 487.5 g of ethanol solution to precipitate hemicellulose. The solid obtained after filtering and washing the mixed liquid is the separated hemicellulose, and the hemicellulose is freeze-dried to obtain hemicellulose powder.

[0077] The product analysis was carried out according to the test method in Example 1.

[0078] According to the analysis results, the hemicellulose yield is 65%, the purity of hemicellulose is 100%, and according to Figure 3 According to the GPC test results, the degree of polymerization of hemicellulose is 220; the degree of polymerization of high-purity cellulose is 965, the ash content is 0.3%, the Fe content is 1 ppm, and the α-cellulose content is 95%. Figure 4 This is the liquid phase spectrum of the high-purity cellulose after hydrolysis into monosaccharides. The hemicellulose content in the high-purity cellulose is calculated to be 1.5% by peak area. Figure 5 This is a transmission electron micrograph of the hemicellulose aqueous dispersion separated in Example 2.

[0079] In addition, the high-purity cellulose pulp obtained in step (5) was dissolved in N-methylmorpholine oxide (NMMO) solvent, and lyocell fibers were obtained by wet spinning. Figure 6 shown.

[0080] Example 3: Preparation of high-polymerization and high-purity hemicellulose by one-step LiBr / CaCl2 molten salt method

[0081] Step (1): Weigh 20 g of bleached softwood pulp, soak the pulp in 10 times the volume of deionized water overnight, mechanically stir at 200 rpm for 20 minutes to loosen the pulp, drain excess water, and freeze-dry the pulp for later use.

[0082] Step (2): Weigh 120 g of LiBr and 10 g of CaCl2, add them to 95 g of deionized water, and mix them evenly with magnetic stirring in a 25°C water bath and stabilize for 30 minutes. The corresponding inorganic salt concentration is 58%.

[0083] Step (3): Weigh 6.75 g of the bone-dry pulp prepared in step (1), add it to the molten salt hydrate prepared in step (2), and stir in a water bath at 55° C. for 3 h. The corresponding pulp concentration is 3%.

[0084] Step (4): Pour the mixture obtained in step (3) into a vacuum filtration device for solid-liquid separation, filter using a polypropylene filter membrane, and collect the filtered solid and liquid separately.

[0085] Step (5): The solid collected in step (4) is washed and desalted to obtain high-purity cellulose.

[0086] Step (6): The liquid collected in step (4) is added to 337.5 g of deionized water to precipitate the hemicellulose. The solid obtained after filtering and washing the mixed liquid is the separated hemicellulose, and the hemicellulose is freeze-dried to obtain hemicellulose powder.

[0087] The product analysis was carried out according to the test method in Example 1.

[0088] According to the analysis results, the hemicellulose yield is 95% and the purity of hemicellulose is 100%. Figure 7 According to the GPC test results, the degree of polymerization of hemicellulose is 230; the degree of polymerization of high-purity cellulose is 1000, the ash content is 0.11%, the Fe content is 2.1ppm, the α-cellulose content is 98%, and the hemicellulose content in the high-purity cellulose is 3%.

[0089] In addition, the high-purity cellulose pulp obtained in step (5) was dissolved in N-methylmorpholine oxide (NMMO) solvent, and then lyocell fibers were obtained by wet spinning.

[0090] Example 4: Preparation of high-polymerization and high-purity hemicellulose by one-step FeCl3 molten salt method

[0091] Step (1): Weigh 20 g of bleached pulp of Herba Eupatorii, place the pulp in a grinder to loosen it into a cotton-like state, and dry the loosened pulp until it is absolutely dry for later use.

[0092] Step (2): Weigh 120 g of FeCl3, add it to 51.4 g of deionized water, and stir it in a 25°C water bath with magnetic stirring to mix it evenly and stabilize it for 30 minutes. The corresponding inorganic salt concentration is 70%.

[0093] Step (3): Weigh 1.714 g of the bone-dry pulp prepared in step (1), add it to the molten salt hydrate prepared in step (2), and stir in a water bath at 55° C. for 5 h. The corresponding pulp concentration is 1%.

[0094] Step (4): Pour the mixture obtained in step (3) into a vacuum filtration device for solid-liquid separation, filter using a polypropylene filter membrane, and collect the filtered solid and liquid separately.

[0095] Step (5): The solid collected in step (4) is washed and desalted to obtain high-purity cellulose.

[0096] Step (6): The liquid collected in step (4) is added to 206 g of acetone solution to precipitate hemicellulose. The solid obtained after filtering and washing the mixed liquid is the separated hemicellulose, and the hemicellulose is freeze-dried to obtain hemicellulose powder.

[0097] The product analysis was carried out according to the test method in Example 1.

[0098] According to the analysis results, the hemicellulose yield is 80%, the hemicellulose purity is 100%, and the polymerization degree is 223; the polymerization degree of high-purity cellulose is 500, the ash content is 0.24%, the Fe content is 5ppm, the α-cellulose content is 93%, and the hemicellulose content in high-purity cellulose is 2.1%.

[0099] In addition, the high-purity cellulose pulp obtained in step (5) was dissolved in N-methylmorpholine oxide (NMMO) solvent, and then lyocell fibers were obtained by wet spinning.

[0100] Example 5: Preparation of high-polymerization and high-purity hemicellulose by one-step LiBr / AlCl3 molten salt method

[0101] Step (1): Weigh 30 g of bleached hardwood pulp, soak the pulp in water for hydraulic pulping, evenly disintegrate the pulp, and then squeeze out excess water until the pulp has a moisture content of 50%.

[0102] Step (2): Weigh 110 g of LiBr and 10 g of AlCl3, add them to 60 g of deionized water, and mix them evenly with magnetic stirring in a 25°C water bath and stabilize for 30 minutes. The corresponding inorganic salt mass fraction is 60%.

[0103] Step (3): Weigh 40 g of the pulp with a humidity of 50% prepared in step (1), add it to the molten salt hydrate prepared in step (2), and stir in a 30°C water bath for 7 h. The corresponding pulp concentration is 10%.

[0104] Step (4): Pour the mixture obtained in step (3) into a vacuum filtration device for solid-liquid separation, filter using a polypropylene filter membrane, and collect the filtered solid and liquid separately.

[0105] Step (5): The solid collected in step (4) is washed and desalted to obtain high-purity cellulose.

[0106] Step (6): The liquid collected in step (4) is added to 200 g of ethanol solution to precipitate hemicellulose. The solid obtained after filtering and washing the mixed liquid is the separated hemicellulose, and the hemicellulose is freeze-dried to obtain hemicellulose powder.

[0107] The product analysis was carried out according to the test method in Example 1.

[0108] According to the analysis results, the hemicellulose yield is 84%, the hemicellulose purity is 100%, and the polymerization degree is 208; the polymerization degree of high-purity cellulose is 913, the ash content is 0.17%, the Fe content is 0.87ppm, the α-cellulose content is 94%, and the hemicellulose content in the high-purity cellulose is 1.7%.

[0109] In addition, the high-purity cellulose pulp obtained in step (5) was dissolved in N-methylmorpholine oxide (NMMO) solvent, and then lyocell fibers were obtained by wet spinning.

[0110] Example 6: Study on the solubilization mechanism of hemicellulose by molten salt hydrate

[0111] The inventors speculate that the solubilization mechanism of molten salt hydrate on hemicellulose is the interaction between metal cations and hemicellulose chains. In order to obtain direct evidence of this interaction, the hemicellulose regenerated in Example 3 was redissolved in molten salt hydrate (LiBr·3H2O) and the hemicellulose-molten salt hydrate liquid was subjected to nuclear magnetic resonance spectroscopy. 7 Li NMR test, the chemical shift of lithium is as follows Figure 8 As shown. Figure 8 It can be seen that when hemicellulose exists in LiBr·3H2O, 7 The Li NMR signal will shift. As the hemicellulose concentration increases, the shielding effect of lithium ions can be observed to increase, and the displacement of lithium ions also increases. This shows that the metal cations in the molten salt hydrate interact with the hemicellulose chains, thereby promoting the dissolution of hemicellulose in the molten salt hydrate. In other words, the molten salt hydrate can efficiently dissolve and separate hemicellulose in pulp.

[0112] Example 7: Performance test of lyocell fibers prepared in Examples 1-5

[0113] The following performance tests were performed on the lyocell fibers prepared in Examples 1-5:

[0114] (1) Fiber linear density test

[0115] The linear density of the fibers was measured using an XD-1 fineness meter.

[0116] (2) Testing of fiber tensile properties

[0117] With reference to GB / T 9997-1988 and GB / T 4711-1984, the dry strength, wet strength, elongation and wet elongation of Lyocell fiber were tested using a YG004 electronic single fiber strength tester.

[0118] (3) Fiber friction coefficient test

[0119] Referring to T / CSTM 00522-2022, five types of Lyocell fibers were placed across a test roller at a specific angle θ (θ = 180°). A load F1 was applied to one end of the fiber, while a force F2 was applied to the other end. Because force F1 was greater than force F2, friction was generated between the fiber and the cylindrical surface of the roller. The static and kinetic coefficients of friction between the fiber and the roller were calculated using Euler's law.

[0120] (4) Fiber regain test

[0121] Referring to the national standards GB / T 6102.1-2006 and GB / T 9995-1997, five types of lyocell fibers were placed under standard conditions to reach moisture absorption equilibrium, and then the fibers were tested for moisture regain in a constant temperature oven.

[0122] (5) Fiber wet expansion test

[0123] With reference to GB / T 10685-2007, Lyocell fibers were soaked in deionized water for different time periods. The dry diameter and wet expansion diameter of the fibers were observed and measured using a fiber fineness meter (magnification 500 times). 30 fibers were measured separately, and the average value was calculated to determine the transverse wet expansion rate of the fibers.

[0124] Calculation formula:

[0125]

[0126] Among them, D w is the fiber diameter in wet state, D d is the fiber diameter in dry state.

[0127] The various performance parameters of the lyocell fibers prepared in Examples 1-5 were obtained through the aforementioned tests, and the results are shown in Table 1.

[0128] Table 1. Performance test results of lyocell fibers prepared in Examples 1-5

[0129] Example 1 Example 2 Example 3 Example 4 Example 5 Linear density (dtex) 1.7 1.6 1.3 1.4 1.5 Dry strength (cN / dtex) 4.2 4 4.4 3.9 4.1 Wet strength (cN / dtex) 3.8 3.6 3.9 3.5 3.7 Elongation (%) 15 19 12 14 14 Wet tensile strength (%) 18 21 18 18 17 Dynamic friction coefficient (%) 4.6 4.3 5.5 4.6 5.1 Static friction coefficient (%) 5.8 5.7 7.0 5.9 6.3 Moisture regain (%) 12.9 13.3 12.4 13.8 13.2 Wet expansion rate (%) 65.3 69.1 61.9 67.1 64.5

[0130] As shown in Table 1, the lyocell fibers prepared in Examples 1-5 of the present application have a linear density of 1.3-1.7 dtex, a dry strength of 3.9-4.2 cN / dtex, a wet strength of 3.5-3.9 cN / dtex, an elongation of 12-19%, a wet elongation of 17-21%, a kinetic friction coefficient of 4.3-5.1%, a static friction coefficient of 5.7-7.0%, a moisture regain of 12.4-13.8%, and a wet expansion coefficient of 61.9-69.1%. This indicates that the performance indicators of these fibers are comparable to those of commercial Tencel, and they are expected to replace commercial Tencel, with broad market application prospects and good economic benefits.

[0131] In summary, Examples 1-5 use the method described in this application to prepare high-polymerization-degree high-purity hemicellulose in one step, and co-produce high-purity cellulose. Among them, the hemicellulose yield is as high as 65-95%, the polymerization degree is as high as 200-230, and the purity is 100%, which solves the problems of low polymer, low purity and low yield in the preparation of hemicellulose in the prior art. The polymerization degree of the co-produced high-purity cellulose is as high as 500-1000, and the α-cellulose content is as high as 92-98%; the lyocell fiber prepared by wet spinning using the high-purity cellulose has excellent performance comparable to commercial Tencel, broad market application prospects, and good economic benefits. In addition, the molten salt hydrate and regenerated solvent used in the preparation method described in this application are recycled and reused after solid-liquid separation, distillation and concentration, and no waste is generated in the reaction. It is not only green and environmentally friendly, but also significantly reduces production costs, which is of great significance for large-scale industrial applications.

[0132] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the description and implementation methods. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and examples shown and described herein.

Claims

1. A method for preparing high-polymerization-degree, high-purity hemicellulose, characterized by: The following steps are involved: (1) Weigh an appropriate amount of bleached pulp raw material and immerse it in a molten salt hydrate at a temperature of 25-55°C for 0.5-7 h; the pulp raw material has a slurry concentration of 1-10 wt% in the molten salt hydrate, and the mass fraction of inorganic salt in the molten salt hydrate system is 40-70 wt%; the molten salt hydrate is Li + Mg 2+ , Ca 2+ 、Zn 2+ 、Al 3+ 、Fe 3+ and Cu 2+ (2) after the impregnation, the solid and liquid are separated, and an appropriate amount of regeneration solvent is added to the liquid phase to wash and desalt the precipitate to obtain hemicellulose with a high degree of polymerization, and the solid phase is directly washed and desalted to obtain high-purity cellulose; the amount of the regeneration solvent used is 1-1.5 times the mass of the molten salt hydrate.

2. The method for preparing high-polymerization-degree, high-purity hemicellulose according to claim 1, characterized in that: The molten salt hydrate is one or more hydrates of zinc chloride, calcium chloride, lithium bromide or ferric chloride.

3. The method for preparing high-polymerization-degree, high-purity hemicellulose according to claim 1, characterized in that: The regeneration solvent is one of water, ethanol, acetone and methanol.

4. The method for preparing high-polymerization-degree, high-purity hemicellulose according to any one of claims 1 to 3, characterized in that: The raw material for bleached pulp is broadleaf pulp, softwood pulp, straw pulp, hemp pulp, bamboo pulp or bagasse pulp.

5. The method for preparing high-polymerization-degree, high-purity hemicellulose according to claim 4, characterized in that: The process also includes the recovery of molten salt hydrate and regenerated solvent, specifically, separating the precipitate in the liquid phase in step (2), separating the remaining liquid phase by distillation to obtain regenerated solvent and salt solution, and then concentrating the salt solution to the initial concentration, which can be reused in step (1).

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