Preparation and application of high polymerization degree and high substitution degree cellulose formate

Abstract

The application provides a preparation method of cellulose formate with high polymerization degree and high substitution degree, and comprises the following steps: impregnating treatment of cellulose raw material in a molten salt hydrate, and then solid-liquid separation; the separated solid phase is cellulose slurry, and the separated liquid phase is molten salt hydrate, which is recycled. The cellulose slurry is dispersed in a formic acid solution for reaction, and then a regeneration solvent is added after the reaction is completed, and then solid-liquid separation, cleaning and drying are carried out, so that cellulose formate with high polymerization degree is obtained; or the cellulose slurry is dispersed in a formic acid solution for reaction, and then direct spinning or film forming is carried out after the reaction is completed. The preparation method has mild conditions, avoids severe degradation of cellulose in the reaction process, and has high polymerization degree and high substitution degree of the obtained cellulose formate, and the yield is as high as 117-135% of the raw material. Meanwhile, the molten salt hydrate and the formic acid reagent used in the preparation method can be recycled, the preparation cost is low, and the preparation method has wide market application prospect and great economic value.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials and relates to a cellulose derivative, specifically to a method for preparing high-polymerization-degree cellulose formate and its application. Background Technology

[0002] Based on the type of acid that reacts with cellulose, cellulose esters can be classified into organic esters and inorganic esters. Cellulose esters are commercially important biomass modifiers with wide applications in plastics, films, fibers, coatings, and pharmaceuticals. In particular, cellulose acetate, cellulose nitrate, and cellulose sulfate are all industrially produced. Furthermore, cellulose esters have shown significant potential for application in the pharmaceutical field.

[0003] Currently, there are few reports on the research and application of cellulose formate. This is mainly because the cellulose formate prepared by existing methods has a low polymerization rate, resulting in poor performance and limited application prospects. Generally speaking, the preparation principle of cellulose formate is as follows: each glucose unit of the cellulose macromolecule contains three polar alcohol hydroxyl groups; in formic acid solution, the alcohol hydroxyl groups undergo a protonation reaction to generate the corresponding cellulose formate. However, due to the anti-degradation barrier effect of natural cellulose, the aforementioned protonation reaction usually requires sulfuric acid, hydrochloric acid, phosphoric acid, or zinc chloride as catalysts. Although this method can yield cellulose formate with a high degree of substitution, the severe degradation of cellulose during hydrolysis results in a low polymerization rate (<200). This further leads to poor mechanical properties when using this cellulose formate to prepare films or other materials (J.Poly.Sci.:Poly.Lett.,1986,24(10),495; J.Poly.Sci.:Poly.Chem.,1986,24,2981), thus hindering its practical application. Furthermore, this method also involves the reaction of cellulose with a strong acid catalyst, resulting in insufficient purity of the product, often a mixture of cellulose formate and other cellulose esters. There are also reports of cellulose and formic acid undergoing direct hydrolysis at high temperatures; however, this results in both high reaction temperatures and low product yields. Literature records that the highest yield of bleached softwood pulp reacting with 98 wt% formic acid at 90°C for 20 hours was only 45% (ACS Sustain. Chem. Eng., 2019, 7, 9449).

[0004] In summary, current techniques for preparing cellulose formate suffer from low polymerization degree, low yield, and difficulty in obtaining pure products, significantly limiting their applications. Obtaining cellulose formate with high polymer content, high degree of substitution, and high yield is a prerequisite for their widespread use. Currently, no methods for preparing high-polymerization-degree cellulose formate with high yield have been reported. Summary of the Invention

[0005] To address the problems existing in the preparation methods of cellulose formate in the prior art, this invention provides a method for preparing cellulose formate with a high degree of polymerization. This preparation method not only operates under mild conditions, avoiding the drastic degradation of cellulose during hydrolysis, but also achieves a high yield of cellulose formate, ranging from 117% to 135% of the raw material. Furthermore, the molten salt hydrate and formic acid reagent used in the preparation method described in this application are reusable, resulting in low preparation costs and broad market application prospects and significant economic value.

[0006] The technical solution of the present invention: a method for preparing high-polymerization degree cellulose formate or its derivatives, comprising the following steps:

[0007] (1) The cellulose raw material is impregnated in a molten brine, followed by solid-liquid separation. The solid phase after separation is a cellulose slurry, which is washed and dried. The liquid phase after separation is the molten brine, which is recycled. The solid content of the cellulose raw material in the molten brine is 1-20 wt%, the impregnation temperature is 10-50℃, and the impregnation time is 1-100 min. The cellulose raw material is microcrystalline cellulose, cotton pulp, dissolving pulp, sea squirt cellulose, or bacterial cellulose. The molten brine is Li + Mg 2 + Ca 2+ Zn 2+ Al 3+ Fe 3+ and Cu 2+ It contains one or two of the following: chloride salts, bromide salts, iodate salts, perchlorate salts, nitrate salts, and thiocyanate hydrates; the inorganic salt weight fraction in the molten salt hydrate is 40-70%.

[0008] This step controls the solid content, temperature conditions, and treatment time of the cellulose raw material during impregnation to change the crystal form of the cellulose raw material without dissolving it. The reaction conditions are mild and the process is simple, which is of great significance for industrial production.

[0009] (2) The cellulose slurry obtained in step (1) is dispersed in a formic acid solution and reacted. After the reaction is complete, a regenerating solvent is added, followed by solid-liquid separation and washing to obtain a high-polymerization-degree cellulose formate wet material; or the cellulose slurry obtained in step (1) is dispersed in a formic acid solution and reacted. After the reaction is complete, it is directly spun or formed into a film. The solid content of the cellulose slurry in the formic acid solution is 1-20 wt%, the concentration of the formic acid solution is 50-98%, the reaction temperature is 10-50℃, and the reaction time is 1h-72h. The regenerating solvent is water, ethanol, acetone, tert-butanol, or n-butanol.

[0010] The alteration of the cellulose structure in the cellulose slurry after step (1) allows it to react with formic acid at a lower temperature in this step, thus avoiding drastic degradation of cellulose during the reaction. This results in the preparation of cellulose formate esters with advantages of high substitution degree (1.0-2.0), high polymerization degree (300-1000), and high yield (117-135% of raw material). Compared with existing cellulose formate esters, the product performance is significantly improved, producing unexpected technical effects. This method solves the difficulties faced in actual production and has important practical application value.

[0011] Preferably, the molten salt hydrate is LiBr·3H2O, ZnCl2·4H2O, LiCl·3H2O, AlCl3·6H2O, FeCl3·6H2O, MgCl2·6H2O, CaCl2·6H2O, LiNO3·3H2O, Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, LiClO4·3H2O, or LiSCN·2.5H2O.

[0012] Preferably, the cellulose slurry in step (1) is washed with water, and the resulting liquid phase is a salt solution. The molten salt hydrate is separated by chromatography or rotary evaporation, concentrated to the initial concentration, and then recycled. The liquid obtained from the solid-liquid separation in step (2) is a formic acid-regenerated solvent solution, which can also be recycled after being concentrated to the initial concentration. In this application, both the molten salt and formic acid can be recycled, which greatly reduces the process cost. The entire process is green, clean, and sustainable, which has important practical significance for promoting the high-value utilization of cellulose.

[0013] The high-polymerization degree cellulose formate prepared by the aforementioned method has a degree of substitution of 1.0-2.0, a cellulose type II crystal form, a degree of polymerization of 300-1000, and a yield of 117-135% of the raw material. Furthermore, the cellulose formate is soluble in formic acid, dimethyl sulfoxide, pyridine, N-methylpyrrolidone, N,N-dimethylformamide, or N,N-dimethylacetamide, facilitating further processing and application. It can also serve as an intermediate for the further preparation of various cellulose derivatives. Simultaneously, the high-substituted cellulose formate exhibits good interfacial compatibility with polymers (rubber, plastics, etc.), which can be used to improve their mechanical properties and thermal stability. Moreover, as a biodegradable regenerated cellulose ester, cellulose formate holds promise as a substitute for petroleum-based materials, which is of significant importance for environmental protection.

[0014] As described above, the high-polymerization-degree cellulose formate is used to prepare cellulose derivatives or cellulose formate-derived products. Specifically, the cellulose formate is dissolved in formic acid, dimethyl sulfoxide, pyridine, N-methylpyrrolidone, N,N-dimethylformamide, or N,N-dimethylacetamide. Then, cellulose derivatives are obtained through a highly reactive aldehyde reaction, high-performance cellulose formate films are obtained through casting, or high-tensile-strength cellulose formate filaments are obtained through spinning. This is because the formate group (i.e., the formyl group) of cellulose formate is also an aldehyde group, which has reducing properties. The aldehyde group can be further oxidized to a carboxyl group, or undergo a Schiff base reaction with an amino-containing compound, or react with sodium bisulfite to introduce a sulfonic acid group. Therefore, by using cellulose formate as an intermediate and further derivatizing it, cellulose products with different properties can be obtained.

[0015] The beneficial effects of this invention are:

[0016] (1) This invention provides a novel method for preparing cellulose formate. The preparation method is not only simple to operate, with mild reaction conditions and low process cost, but also has a high product yield (117-135% of raw materials), high degree of substitution (1.0-2.0), and high degree of polymerization (300-1000), which solves the bottleneck in actual production and has important practical application value.

[0017] (2) The preparation method described in this invention includes two steps: pre-impregnation of cellulose in a molten salt hydrate system and hydrolysis with formic acid. No other auxiliaries (sulfuric acid, phosphoric acid, hydrochloric acid, etc.) are required. The resulting product has high purity and no by-products. Furthermore, both the molten salt hydrate and formic acid can be recycled using simple methods, resulting in low process costs.

[0018] (3) The cellulose formate obtained by the preparation method described in this invention has a high degree of polymerization and can be directly cast to obtain a high-strength transparent cellulose formate film. The tensile strength of the film can reach 75-100MPa, the light transmittance can reach 91-96% (550nm), and it has good barrier properties. It can also be spun to obtain a high-strength cellulose formate filament. The tensile strength of the filament can reach 1.0-1.5cN / dtex.

[0019] (4) The cellulose formate obtained by the preparation method described in this invention is soluble in organic solvents such as formic acid, dimethyl sulfoxide, pyridine, N-methylpyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide. Its maximum solubility in formic acid can reach 10%, and its maximum solubility in dimethyl sulfoxide, pyridine, N-methylpyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide can reach 8%. It is convenient to use highly reactive aldehyde groups to prepare cellulose derivatives and has broad application prospects. Attached Figure Description

[0020] Appendix Figure 1 This is a photograph of the cellulose formate prepared in Example 1.

[0021] Appendix Figure 2 This is a photograph of the cellulose formate film prepared in Example 1.

[0022] Appendix Figure 3 This is a stress-strain diagram of the cellulose formate film prepared in Example 1.

[0023] Appendix Figure 4 This is a transmittance diagram of the cellulose formate film prepared in Example 1.

[0024] Appendix Figure 5 This is the spinning process of cellulose formate as described in Example 3.

[0025] Appendix Figure 6 This is a photograph of the cellulose formate filaments prepared in Example 3. Detailed Implementation

[0026] The present invention will be further described below with reference to the embodiments.

[0027] The present invention will be further described below with reference to embodiments. The feasibility of the method is further described through specific implementation examples, but this does not mean that the present invention is limited to these examples.

[0028] Example 1: Preparation of high-polymerization, high-substitution-degree cellulose formate

[0029] First, cotton pulp is pre-impregnated in a molten briquette solution. The cotton pulp has a solid content of 1%, and the molten briquette solution is a 60% LiBr solution. After impregnation at 50°C for 5 minutes, the pulp is washed and dried. The recovered molten briquette solution is then recycled after rotary evaporation. Next, the pre-impregnated pulp is added to a 98% formic acid solution for reaction. The cotton pulp has a solid content of 1%. The reaction temperature is 30°C, and the reaction time is 72 hours. After the reaction, washing with water yields a wet cellulose formate stock, such as... Figure 1 As shown, it is a white paste at room temperature. The formic acid, after washing, is reused after rotary evaporation.

[0030] Characterization results show that the obtained cellulose formate has a degree of substitution of 2.0, a degree of polymerization of 600, and a yield of 135% of the raw material. Furthermore, directly casting a formic acid solution of the obtained cellulose formate yields a high-strength, transparent film, such as... Figure 2 As shown, the patterns and text beneath the film are clearly visible. Testing revealed that the tensile strength of the film is 100 MPa. Figure 3 ), transmittance is 96% (550nm, Figure 4 It also has good barrier properties (oxygen permeability coefficient 3.838E-11, moisture permeability 4.57g / (m³)). 2 (·h), with a water contact angle of 85°. Furthermore, this cellulose formate exhibits a solubility of up to 10% in anhydrous formic acid and a maximum solubility of up to 8% in dimethyl sulfoxide.

[0031] Comparative Example 1: Changing the immersion temperature

[0032] First, cotton pulp was pre-impregnated in a molten briquette solution. The cotton pulp had a solid content of 1%, and the molten briquette was a 60% LiBr solution. After impregnation at 90°C for 5 minutes, the pulp was washed and dried. The recovered molten briquette was then recycled after rotary evaporation. Next, the pre-impregnated pulp was added to a 98% formic acid solution. The cotton pulp had a solid content of 1%. The reaction temperature was 30°C, and the reaction time was 72 hours. After the reaction, the pulp was washed with water to obtain a wet cellulose formate. The washed formic acid was then recycled after rotary evaporation.

[0033] The characterization results show that the obtained cellulose formate has a degree of substitution of 1.2, a degree of polymerization of 100, and a yield of 85% of the raw material. Furthermore, directly casting the formic acid solution of the obtained cellulose formate does not yield a high-strength film. This indicates that excessively high temperatures during molten salt hydrate pretreatment prevent the production of cellulose formate with high degrees of substitution, high degree of polymerization, and high yield. This is because partial degradation of cellulose leads to a decrease in the degree of polymerization and yield of the cellulose formate.

[0034] Comparative Example 2: Changing the reaction temperature of formic acid

[0035] First, cotton pulp is pre-impregnated in a molten briquette solution. The cotton pulp has a solid content of 1%, and the molten briquette solution is a 60% LiBr solution. After impregnation at 50°C for 5 minutes, the pulp is washed and dried. The recovered molten briquette solution is then recycled after rotary evaporation. Next, the pre-impregnated pulp is added to a 98% formic acid solution. The cotton pulp has a solid content of 1%. The reaction temperature is 60°C, and the reaction time is 1 hour. After the reaction, the pulp is washed with water to obtain a wet cellulose formate. The washed formic acid is then recycled after rotary evaporation.

[0036] The characterization results show that the obtained cellulose formate has a degree of substitution of 2.1, but a degree of polymerization of only 50, with a yield of 70% of the raw material. Furthermore, due to the low degree of polymerization of the obtained cellulose formate, direct casting with it cannot produce a film. This indicates that the reaction temperature of the molten salt pretreated slurry in formic acid is too high (above 50°C), which cannot yield cellulose formate with a high degree of polymerization and high yield.

[0037] Example 2: Preparation of high-polymerization-degree, high-substitution-degree cellulose formate

[0038] First, microcrystalline cellulose was pre-impregnated in a molten brine solution. The cellulose solids content was 20%, and the molten brine solution was a 70% ZnCl2 solution. After impregnation at 10°C for 1 minute, the slurry was washed and dried, and the recovered molten brine solution was reused after rotary evaporation. Then, the pre-impregnated slurry was added to a 50% formic acid solution for reaction. The cotton pulp solids content was 20%. The reaction temperature was 10°C, and the reaction time was 1 hour. After the reaction, washing yielded a wet cellulose formate ester, and the washed formic acid was reused after rotary evaporation.

[0039] Characterization results showed that the obtained cellulose formate had a degree of substitution of 1.0, a degree of polymerization of 300, and a yield of 117%. Furthermore, a high-strength film could be obtained by directly casting the formic acid solution of the obtained cellulose formate. Testing revealed that the film had a tensile strength of 75 MPa, a light transmittance of 92% (550 nm), and good barrier properties (oxygen permeability coefficient 8.037E-10, moisture permeability 3.96 g / (m²)). 2 (·h), with a water contact angle of 80°. Furthermore, this cellulose formate exhibits a solubility of up to 8% in N-methylpyrrolidone.

[0040] Example 3: Preparation of high-polymerization, high-substitution-degree cellulose formate

[0041] First, the hardwood dissolving pulp was pre-impregnated in a molten brine solution. The pulp solids content was 10%, and the molten brine solution was a 40% ZnCl2 solution. After impregnation at 40°C for 10 minutes, the pulp was washed and dried, and the recovered molten brine solution was reused after rotary evaporation. Then, the pre-impregnated pulp was added to an 88% formic acid solution. The pulp solids content was 5%. The reaction temperature was 50°C, and the reaction time was 48 hours. After the reaction, the pulp was washed and dried to obtain a wet cellulose formate ester. The washed formic acid was reused after rotary evaporation.

[0042] The characterization results show that the obtained cellulose formate has a degree of substitution of 1.4, a degree of polymerization of 500, and a yield of 124%. Furthermore, the obtained cellulose formate solution was spun in a 5% NaOH solution, and the spinning process is as follows: Figure 5 As shown, after washing and drying, cellulose formate filaments can be obtained, such as... Figure 6 As shown. The tensile strength of this filament was tested to be 1.5 cN / dtex. Furthermore, the cellulose formate exhibits a solubility of up to 8% in N,N-dimethylformamide and N,N-dimethylacetamide.

[0043] Example 4: Preparation of high-polymerization, high-substitution-degree cellulose formate

[0044] First, the softwood dissolving pulp was pre-impregnated in a molten salt hydrate. The pulp had a solids content of 15%, and the molten salt hydrate contained 40% LiBr and 10% CaCl2. After impregnation at 25°C for 80 minutes, the pulp was washed and dried, and the recovered molten salt hydrate was reused after rotary evaporation. Then, the pre-impregnated pulp was added to a 60% formic acid solution with a solids content of 12%. The reaction temperature was 10°C, and the reaction time was 40 hours. After the reaction, washing yielded a wet cellulose formate stock, and the washed formic acid was reused after rotary evaporation.

[0045] Characterization results showed that the obtained cellulose formate had a degree of substitution of 1.8, a degree of polymerization of 450, and a yield of 129%. Furthermore, a high-strength film could be obtained by directly casting the formic acid solution of the obtained cellulose formate. Testing revealed that the film had a tensile strength of 89 MPa, a light transmittance of 95% (550 nm), and good barrier properties (oxygen permeability coefficient 2.938E-11, moisture permeability 5.28 g / (m²)). 2 (·h), with a water contact angle of 90°. Furthermore, this cellulose formate exhibits a solubility of up to 10% in anhydrous formic acid and up to 8% in pyridine.

[0046] Example 5: Preparation of high-polymerization, high-substitution-degree cellulose formate

[0047] First, sea squirt cellulose was pre-impregnated in a molten brine solution. The slurry had a solids content of 5%, and the molten brine solution contained 40% AlCl3 and 10% ZnCl2. After impregnation at 20°C for 50 minutes, the slurry was washed and dried, and the recovered molten brine solution was reused after rotary evaporation. Then, the pre-impregnated slurry was added to a 70% formic acid solution with a solids content of 8%. The reaction temperature was 15°C, and the reaction time was 36 hours. After the reaction, washing yielded a wet cellulose formate ester, and the formic acid was reused after rotary evaporation.

[0048] Characterization results showed that the obtained cellulose formate had a degree of substitution of 1.2, a degree of polymerization of 350, and a yield of 121%. Furthermore, the obtained cellulose formate solution was spun in water and dried to obtain cellulose formate filaments. The tensile strength of these filaments was tested to be 1.0 cN / dtex. In addition, the cellulose formate exhibited a solubility of up to 8% in anhydrous formic acid and up to 6% in N-methylpyrrolidone.

[0049] Example 6: Preparation of high-polymerization, high-substitution-degree cellulose formate

[0050] First, bacterial cellulose was pre-impregnated in a molten brine solution. The slurry had a solids content of 2%, and the molten brine solution contained 30% LiCl and 40% ZnCl2. After impregnation at 15°C for 90 minutes, the slurry was washed and dried, and the recovered molten brine solution was reused after rotary evaporation. Then, the pre-impregnated slurry was added to an 80% formic acid solution with a solids content of 15%. The reaction temperature was 40°C, and the reaction time was 5 hours. After the reaction, washing yielded a wet cellulose formate ester, and the formic acid was reused after rotary evaporation.

[0051] Characterization results showed that the obtained cellulose formate had a degree of substitution of 1.0, a degree of polymerization of 300, and a yield of 117%. Furthermore, the obtained cellulose formate solution was spun in acetone solution, and after washing and drying, cellulose formate filaments were obtained. The tensile strength of these filaments was tested to be 1.1 cN / dtex. In addition, the cellulose formate exhibited a solubility of up to 5% in N-methylpyrrolidone and up to 6% in N,N-dimethylacetamide.

[0052] Example 7: Preparation of high-polymerization, high-substitution-degree cellulose formate

[0053] First, the softwood dissolving pulp was pre-impregnated in a molten brine solution. The pulp had a solids content of 1%, and the molten brine solution contained 10% LiBr and 30% MgCl2. After impregnation at 30°C for 100 min, the pulp was washed and dried, and the recovered molten brine solution was reused after rotary evaporation. Then, the pre-impregnated pulp was added to a 60% formic acid solution. The pulp had a solids content of 10%. The reaction temperature was 50°C, and the reaction time was 60 h. After the reaction, washing yielded a wet cellulose formate stock, and the washed formic acid was reused after rotary evaporation.

[0054] Characterization results showed that the obtained cellulose formate had a degree of substitution of 1.9, a degree of polymerization of 500, and a yield of 132%. Furthermore, a high-strength film could be obtained by directly casting the formic acid solution of the obtained cellulose formate. Testing revealed that the film had a tensile strength of 95 MPa, a light transmittance of 91% (550 nm), and good barrier properties (oxygen permeability coefficient 9.378E-11, moisture permeability 4.01 g / (m²)). 2 (·h), with a water contact angle of 88°. Furthermore, this cellulose formate exhibits a solubility of up to 10% in N,N-dimethylformamide and up to 8% in pyridine.

[0055] Example 8: Preparation of high-polymerization, high-substitution-degree cellulose formate

[0056] First, cotton pulp was pre-impregnated in a molten salt solution. The cotton pulp had a solid content of 10%, and the molten salt solution contained 20% ZnCl2 and 40% LiClO4. After impregnation at 40°C for 40 minutes, the pulp was washed and dried, and the recovered molten salt solution was reused after rotary evaporation. Then, the pre-impregnated pulp was added to a 98% formic acid solution. The cotton pulp had a solid content of 2%. The reaction temperature was 30°C, and the reaction time was 10 hours. After the reaction, washing yielded a wet cellulose formate ester. The formic acid from the washing was reused after rotary evaporation.

[0057] Characterization results showed that the obtained cellulose formate had a degree of substitution of 2.0, a degree of polymerization of 1000, and a yield of 135%. Furthermore, the obtained cellulose formate solution was spun in anhydrous ethanol, and after washing and drying, cellulose formate filaments were obtained. The tensile strength of these filaments was tested to be 1.4 cN / dtex. In addition, the cellulose formate exhibited a solubility of 8% in dimethyl sulfoxide and 8% in N,N-dimethylacetamide.

[0058] In summary, Examples 1-8, prepared under the mild conditions described in this application, achieve a degree of substitution of 1.0-2.0, a yield of 117-135% of the raw material, and a degree of polymerization of 300-1000. Compared with existing preparation methods, these parameters represent significant improvements. High-strength transparent cellulose formate films can be obtained through direct casting using the described cellulose formate, exhibiting tensile strength of 75-100 MPa, light transmittance of 91-96% (550 nm), and good barrier properties. High-strength cellulose formate filaments can also be obtained through spinning, achieving a tensile strength of 1.0-1.5 cN / dtex. Furthermore, both the films and filaments obtained using cellulose formate possess excellent water resistance and are expected to replace cellulose acetate, showing broad application prospects in membrane materials, pharmaceuticals, and textiles.

[0059] Moreover, compared with the prior art, the preparation method described in this application does not require the use of other additives (e.g., sulfuric acid, phosphoric acid, hydrochloric acid, etc.), the obtained product has no by-products, and both the molten salt hydrate and formic acid can be recycled by simple methods. The process has low cost and has achieved unexpected results, which is of great significance to industrial production.

Claims

1. A method for preparing a high-polymerization-degree, high-substitution-degree cellulose formate, characterized in that: Includes the following steps: (1) The cellulose raw material is impregnated in molten salt hydrate and then the solid and liquid phases are separated. The solid phase after separation is cellulose slurry, which is washed and dried. The liquid phase after separation is molten salt hydrate, which is recycled. The solid content of the cellulose raw material in the molten salt hydrate is 1-20 wt%, the impregnation temperature is 10-30℃, the impregnation time is 1-100 min, and the weight fraction of inorganic salts in the molten salt hydrate is 40-70%. (2) The cellulose slurry obtained in step (1) is dispersed in formic acid solution and reacted. After the reaction is completed, water is added for washing to obtain high degree of polymerization cellulose formate wet material. Alternatively, the cellulose slurry obtained in step (1) can be dispersed in a formic acid solution for reaction. After the reaction is complete, it can be directly spun or formed into a film. The solid content in the formic acid solution is 1-20wt%, the concentration of the formic acid solution is 50-98%, the reaction temperature is 10-50℃, and the reaction time is 1h-72h.

2. The preparation method according to claim 1; characterized in that: The molten salt hydrate is Li + Mg 2+ Ca 2+ Zn 2+ Al 3+ Fe 3+ and Cu 2+ It is one or two of the hydrates of chlorides, bromides, iodates, perchlorates, nitrates, and thiocyanates.

3. The preparation method according to claim 2; characterized in that: The molten salt hydrates mentioned are LiBr·3H2O, ZnCl2·4H2O, LiCl·3H2O, AlCl3·6H2O, FeCl3·6H2O, MgCl2·6H2O, CaCl2·6H2O, LiNO3·3H2O, Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, LiClO4·3H2O, or LiSCN·2.5H2O.

4. The preparation method according to claim 1; characterized in that: The cellulose raw materials mentioned are microcrystalline cellulose, cotton pulp, dissolving pulp, sea squirt cellulose, or bacterial cellulose.

5. The preparation method according to any one of claims 1-4, characterized in that: The cellulose slurry described in step (1) is washed with water, and the resulting liquid phase is a salt solution. This solution is then concentrated to its initial concentration and reused.

6. A highly polymerized, highly substituted cellulose formate prepared using any one of claims 1-5.

7. The high degree of polymerization and high degree of substitution cellulose formate according to claim 6, characterized in that: The degree of substitution of the cellulose formate is 1.0-2.0, the crystal form is cellulose type II, and the degree of polymerization is 300-1000; the cellulose formate is soluble in formic acid, dimethyl sulfoxide, pyridine, N-methylpyrrolidone, N,N-dimethylformamide or N,N-dimethylacetamide.

8. The application of the high degree of polymerization and high degree of substitution cellulose formate as described in claim 6 or 7, characterized in that: The cellulose formate is used to prepare cellulose formate derivatives.

9. The application of the high degree of polymerization and high degree of substitution cellulose formate according to claim 6 or 7, characterized in that: Specifically, the cellulose formate is dissolved in formic acid, dimethyl sulfoxide or pyridine, and then a cellulose formate film is obtained by casting or a cellulose formate filament is obtained by spinning.

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