A deep eutectic solvent for dissolving cellulose and a method for dissolving cellulose using the same

Abstract

The present invention belongs to the technical field of cellulose dissolution. The present invention provides a deep eutectic solvent for dissolving cellulose, and the deep eutectic solvent comprises a hydrogen bond donor and a hydrogen bond acceptor; the molar ratio of the hydrogen bond donor to the hydrogen bond acceptor is 1:0.5 to 3; the hydrogen bond donor is a phenolic compound, and the phenolic compound comprises one or more of phenol, resorcinol, hydroquinone, cresol, thymol and bisphenol A. The present invention also provides a method for dissolving cellulose with the deep eutectic solvent. The present invention provides a method for introducing local chemical bonds between the deep eutectic solvent and cellulose, significantly improving the degree of hydrogen bond disruption of cellulose and the reconstruction efficiency of hydrogen bonds between cellulose and the deep eutectic solvent; the dissolution ability of the deep eutectic solvent of the present invention for cellulose is significantly improved, and the dissolution method of the present invention has high efficiency, short time and is easy to control.

Description

Technical Field

[0001] The present invention relates to the technical field of cellulose dissolution, and in particular to a deep eutectic solvent for dissolving cellulose and a method for dissolving cellulose with the same. Background Art

[0002] Cellulose is a renewable biomass resource with extremely rich reserves and wide sources in nature. Its development and utilization in the fields of energy, chemicals, and materials can effectively alleviate the dependence on non-renewable resources and is of great significance for the development of a sustainable society. However, cellulose has a unique stereochemical structure, strong hydrogen bonding, high degree of polymerization and crystallinity, which makes the energy required for the disentanglement and separation of cellulose molecular chains very high. As a result, cellulose is difficult to dissolve in most solvents and cannot be melt-processed below its thermal decomposition temperature, making cellulose far from being fully utilized on a large scale. Therefore, the development of cellulose solvents and the improvement of their dissolution efficiency have become the key links in the efficient utilization of cellulose resources.

[0003] Currently, novel cellulose solvents with low price, environmental friendliness, simple process, and excellent dissolution performance are the core factors affecting the market competitiveness of cellulose. Common ones include aqueous solvent systems (sodium hydroxide / water, sodium hydroxide / urea, quaternary ammonium salt / quaternary phosphonium salt aqueous solutions and their composite solvents, and molten inorganic hydrates) and organic solvent systems (lithium chloride / N,N-dimethylacetamide, ionic liquids and their composite solvents, deep eutectic solvent systems). Different solvent systems have different advantages and disadvantages. However, considering aspects such as price, safety, recycling ability, dissolution process, and solubility, no solvent has been able to meet the above requirements comprehensively to better meet the industrial development of cellulose dissolution. Deep eutectic solvents (DESs) have the characteristics of simple preparation, low cost, low toxicity, low volatility, non-flammability, and good solubility, and have shown good application prospects in many fields such as electrodeposition, extraction separation, catalytic reaction, and nanomaterial preparation in recent years. They have also been widely studied by domestic and foreign scholars in the field of cellulose dissolution. However, the poor dissolution ability of DESs for cellulose restricts their large-scale development in the field of cellulose materials. From current research, it can be known that the reason for the poor dissolution ability of DESs for cellulose is that the hydrogen bonding in the solvent system itself is relatively strong, and its ability to break the hydrogen bonds of cellulose and form new hydrogen bonds with cellulose molecules is relatively weak.

[0004] Therefore, it is of great economic and social value to research and develop a deep eutectic solvent to enhance the hydrogen bond reconstruction efficiency between the solvent and cellulose and improve the cellulose dissolution ability and dissolution efficiency. Summary of the Invention

[0005] The object of the present invention is to provide a deep eutectic solvent for dissolving cellulose and a method for dissolving cellulose to overcome the deficiencies of the prior art. The present invention provides a method for introducing local chemical bonds between the deep eutectic solvent and cellulose, significantly improving the degree of hydrogen bond disruption of cellulose and the reconstruction efficiency of hydrogen bonds with the deep eutectic solvent. The dissolution method of the present invention is highly efficient, short in time, and easy to control.

[0006] To achieve the above object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a deep eutectic solvent for dissolving cellulose, the deep eutectic solvent comprising a hydrogen bond donor and a hydrogen bond acceptor; the molar ratio of the hydrogen bond donor to the hydrogen bond acceptor is 1:0.5 to 3;

[0008] The hydrogen bond donor is a phenolic compound, the phenolic compound comprising one or more of phenol, resorcinol, hydroquinone, cresol, thymol, and bisphenol A.

[0009] Preferably, the hydrogen bond acceptor comprises one or more of choline chloride, tetramethylammonium chloride, betaine, and methyltriphenylphosphonium bromide; the deep eutectic solvent is obtained by mixing the hydrogen bond donor and the hydrogen bond acceptor; the temperature of the mixing is 60 to 100 °C, and the time is 0.5 to 2 h.

[0010] The present invention also provides a method for dissolving cellulose with the deep eutectic solvent as described above, comprising the following steps:

[0011] 1) Mix cellulose, an oxidant, and water and carry out an oxidation reaction to obtain oxidized cellulose;

[0012] 2) Mix the oxidized cellulose and the deep eutectic solvent to obtain a transparent cellulose solution.

[0013] Preferably, the cellulose in step 1) comprises one or more of cellulose extracted from plants, microcrystalline cellulose, and α-cellulose; the oxidant is periodate and / or ammonium cerium nitrate.

[0014] Preferably, the plant is a herbaceous plant or a woody plant; the periodate is sodium periodate or potassium periodate.

[0015] Preferably, the oxidation rate of the oxidized cellulose is 5 to 50%.

[0016] Preferably, the mass ratio of the cellulose, the oxidant, and water in step 1) is 1:0.4 to 1:5 to 20, the temperature of the oxidation reaction is 25 to 75 °C, and the time is 4 to 24 h.

[0017] Preferably, the mass ratio of the oxidized cellulose to the deep eutectic solvent in step 2) is 0.05 to 0.35:1.

[0018] Preferably, the temperature of the mixing in step 2) is 60-120°C and the time is 0.5-4 h; the mixing is carried out under the conditions of stirring, ultrasonic treatment, internal mixing, open mixing or extrusion processing.

[0019] The beneficial effects of the present invention are as follows:

[0020] The eutectic solvent of the present invention weakens its own hydrogen bond interaction, significantly improves the degree of destruction of cellulose hydrogen bonds and the ability to form new hydrogen bonds with cellulose molecules, and the solubility of the eutectic solvent of the present invention in cellulose is significantly improved. Description of the Drawings

[0021] Figure 1 Schematic diagram of the process of dissolving cellulose with the eutectic solvent of Example 1;

[0022] Figure 2 Schematic diagram of the nanocellulose hydrogel and nanocellulose suspension of Example 6;

[0023] Figure 3 Schematic diagram of the thermoplastic composite of Example 7;

[0024] Figure 4 Schematic diagram of the changes in the physical objects of cellulose dissolution in Examples 1-5 and Comparative Examples 1 and 4, where MCC is cellulose and DAC is dialdehyde cellulose obtained by oxidizing cellulose. Detailed Description of the Invention

[0025] The present invention provides a eutectic solvent for dissolving cellulose, and the eutectic solvent contains a hydrogen bond donor and a hydrogen bond acceptor; the molar ratio of the hydrogen bond donor to the hydrogen bond acceptor is 1:0.5-3;

[0026] The hydrogen bond donor is a phenolic compound, and the phenolic compound includes one or more of phenol, resorcinol, hydroquinone, cresol, thymol and bisphenol A.

[0027] Preferably, the molar ratio of the hydrogen bond donor to the hydrogen bond acceptor in the present invention is 1:1-2, and more preferably 1:1.

[0028] When the phenolic compound in the present invention contains several components at the same time, each component is preferably mixed in an equimolar ratio.

[0029] The hydrogen bond acceptor in the present invention preferably comprises one or more of choline chloride, tetramethylammonium chloride, betaine, and methyltriphenylphosphonium bromide; when the hydrogen bond acceptor contains several components simultaneously, the components are preferably mixed in an equimolar ratio; the deep eutectic solvent is preferably obtained by mixing a hydrogen bond donor and a hydrogen bond acceptor; the temperature of the mixing is preferably 60-100°C, more preferably 70-90°C, and even more preferably 80°C; the mixing time is preferably 0.5-2 h, more preferably 0.75-1.75 h, and even more preferably 1-1.5 h.

[0030] The present invention also provides a method for dissolving cellulose with the deep eutectic solvent as described above, comprising the following steps:

[0031] 1) Mix cellulose, an oxidant, and water and then carry out an oxidation reaction to obtain oxidized cellulose;

[0032] 2) Mix the oxidized cellulose and the deep eutectic solvent to obtain a transparent cellulose solution.

[0033] The cellulose in step 1) of the present invention preferably comprises one or more of cellulose extracted from plants, microcrystalline cellulose, and α-cellulose, more preferably microcrystalline cellulose or α-cellulose; the plant for extracting cellulose is preferably a herbaceous plant or a woody plant.

[0034] The oxidant in step 1) of the present invention is preferably periodate and / or ammonium cerium nitrate. When the oxidant contains both periodate and ammonium cerium nitrate, the mass ratio of periodate to ammonium cerium nitrate is preferably 1-3:1, more preferably 2:1; the periodate is preferably sodium periodate or potassium periodate.

[0035] The oxidation rate of the oxidized cellulose in the present invention is preferably 5-50%, more preferably 10-40%, and even more preferably 20-30%.

[0036] The oxidized cellulose in the present invention is mildly oxidized. The oxidation reaction introduces aldehyde groups onto the cellulose molecular chain, and the oxidation position is partial sites of the cellulose molecular chain, preferably the amorphous region.

[0037] The mass ratio of the cellulose, the oxidant, and water in step 1) of the present invention is preferably 1:0.4-1:5-20, more preferably 1:0.6-0.9:8-15, and even more preferably 1:0.7-0.8:10-12; the temperature of the oxidation reaction is preferably 25-75°C, more preferably 35-65°C, and even more preferably 45-55°C; the time of the oxidation reaction is preferably 4-24 h, more preferably 8-20 h, and even more preferably 12-15 h.

[0038] In step 2) of the present invention, the mass ratio of the oxidized cellulose to the deep eutectic solvent is preferably 0.05 - 0.35:1, more preferably 0.1 - 0.25:1, and still more preferably 0.15 - 0.2:1.

[0039] In step 2) of the present invention, the temperature of the mixing is preferably 60 - 120°C, more preferably 70 - 100°C, and still more preferably 80 - 90°C; the mixing time is preferably 0.5 - 4 h, more preferably 1 - 3 h, and still more preferably 2 h; the mixing is preferably carried out under stirring conditions, continuously agitating the solid-liquid mixture to ensure a better mass transfer process; when the mass ratio of the oxidized cellulose to the deep eutectic solvent is 0.05 - 0.15:1, the mixing is preferably carried out under mechanical stirring conditions; when the mass ratio of the oxidized cellulose to the deep eutectic solvent is 0.15 - 0.35:1, the mixing is preferably carried out under ultrasonic, internal mixer, open mill or extrusion processing conditions.

[0040] The method for dissolving cellulose with the deep eutectic solvent of the present invention can be used for preparing nanocellulose, developing nanomaterials or cellulose-based thermoplastic modification; the cellulose-based thermoplastic modification specifically refers to increasing the proportion of cellulose, and using the deep eutectic solvent as a plasticizer to achieve the thermoplastic modification of cellulose in a processing device.

[0041] After the cellulose of the present invention is dissolved, it is preferably regenerated by an aqueous phase, and the deep eutectic solvent is extracted, recovered and recycled, and at the same time, a nanocellulose hydrogel is prepared.

[0042] The technical solutions provided by the present invention will be described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0043] Example 1

[0044] Mix microcrystalline cellulose, sodium periodate and deionized water with a mass ratio of 1:0.6:10, oxidize the microcrystalline cellulose at 50°C for 8 h, and after the oxidation is completed, wash and filter with deionized water 5 times until the reaction reagents are completely removed to obtain oxidized cellulose. Mix resorcinol and choline chloride with a molar ratio of 1:1 at 80°C for 1 h to obtain a transparent and homogeneous deep eutectic solvent at room temperature. Add 1.5 g of oxidized cellulose (absolute dry weight) to 30 g of the deep eutectic solvent, and stir and dissolve at 80°C for 2 h to obtain a cellulose solution.

[0045] The process schematic diagram of dissolving cellulose with the deep eutectic solvent in Example 1 is as Figure 1 shown.

[0046] Example 2

[0047] Mix microcrystalline cellulose, sodium periodate, and deionized water at a mass ratio of 1:0.4:8, oxidize the microcrystalline cellulose at 30 °C for 22 h, and after the oxidation is completed, wash and filter with deionized water 5 times until the reaction reagents are completely removed to obtain oxidized cellulose. Mix phenol and betaine at a molar ratio of 1:0.5 at 60 °C for 2 h to obtain a transparent and homogeneous deep eutectic solvent at room temperature. Add 3 g of oxidized cellulose (absolute dry weight) to 60 g of the deep eutectic solvent, and stir and dissolve at 100 °C for 1 h to obtain a cellulose solution.

[0048] Example 3

[0049] Mix α-cellulose, potassium periodate, and deionized water at a mass ratio of 1:0.6:12, oxidize the microcrystalline cellulose at 70 °C for 4 h, and after the oxidation is completed, wash and filter with deionized water 5 times until the reaction reagents are completely removed to obtain oxidized cellulose. Mix a hydrogen bond donor and a hydrogen bond acceptor at a molar ratio of 1:3 at 100 °C for 0.5 h to obtain a transparent and homogeneous deep eutectic solvent at room temperature. In the hydrogen bond donor, the molar ratio of hydroquinone to cresol is 1:1, and in the hydrogen bond acceptor, the molar ratio of tetramethylammonium chloride to methyltriphenylphosphonium bromide is 1:1. Add 4.5 g of oxidized cellulose (absolute dry weight) to 30 g of the deep eutectic solvent, and react and dissolve in a mixer at 70 °C for 4 h to obtain a cellulose solution.

[0050] Example 4

[0051] Mix cellulose extracted from herbaceous plants, an oxidant (the mass ratio of sodium periodate to ammonium cerium nitrate is 2:1), and deionized water at a mass ratio of 1:0.8:15, oxidize the microcrystalline cellulose at 60 °C for 6 h, and after the oxidation is completed, wash and filter with deionized water 5 times until the reaction reagents are completely removed to obtain oxidized cellulose. Mix a hydrogen bond donor and a hydrogen bond acceptor at a molar ratio of 1:2 at 70 °C for 1 h to obtain a transparent and homogeneous deep eutectic solvent at room temperature. In the hydrogen bond donor, the molar ratio of thymol to bisphenol A is 1:2, and in the hydrogen bond acceptor, the molar ratio of tetramethylammonium chloride to betaine is 2:1. Add 3 g of oxidized cellulose (absolute dry weight) to 15 g of the deep eutectic solvent, and ultrasonically dissolve at 80 °C for 1.5 h to obtain a cellulose solution.

[0052] Example 5

[0053] Mix microcrystalline cellulose, ammonium cerium nitrate, and deionized water at a mass ratio of 1:0.8:9, oxidize the microcrystalline cellulose at 40 °C for 12 h, and after the oxidation is completed, wash and filter with deionized water 5 times until the reaction reagents are completely removed to obtain oxidized cellulose. Mix bisphenol A and betaine at a molar ratio of 1:1.5 at 90 °C for 1 h to obtain a transparent and homogeneous deep eutectic solvent at room temperature. Add 3 g of oxidized cellulose (absolute dry weight) to 60 g of the deep eutectic solvent, and stir and dissolve at 70 °C for 1 h to obtain a cellulose solution.

[0054] Example 6

[0055] Cool the cellulose solution obtained in Example 1 to room temperature, place it in water for solvent replacement to prepare a nanocellulose hydrogel, and ultrasonically disperse the nanocellulose hydrogel in deionized water with an ultrasonic cell disruptor for 0.5 h to obtain a nanocellulose suspension.

[0056] The nanocellulose hydrogel and nanocellulose suspension of Example 6 are as Figure 2 shown, where the nanocellulose hydrogel is on the left and the nanocellulose suspension is in the test tube on the right.

[0057] Example 7

[0058] Mix microcrystalline cellulose, sodium periodate and deionized water with a mass ratio of 1:0.6:10, oxidize the microcrystalline cellulose at 50 °C for 6 h, and after the oxidation is completed, wash and filter with deionized water 5 times until the reaction reagents are removed completely to obtain oxidized cellulose. Mix resorcinol and choline chloride with a molar ratio of 1:1 at 80 °C for 1 h to obtain a transparent and homogeneous eutectic solvent at room temperature. Add 45 g of oxidized cellulose (absolute dry weight) to 30 g of the eutectic solvent, and shear and knead on an open mill at 90 °C for 1 h. The eutectic solvent fully penetrates between cellulose molecules and forms a good bond, and a thermoplastic composite material of eutectic solvent-plasticized cellulose can be formed. The cellulose plasticized region serves as a continuous-phase thermoplastic matrix, and the cellulose crystal region serves as a reinforcing phase of the composite material.

[0059] The thermoplastic composite material of Example 7 is as Figure 3 shown.

[0060] Comparative Example 1

[0061] The mass ratio of microcrystalline cellulose to sodium periodate is 1:0.3, and the other conditions are the same as in Example 1.

[0062] Comparative Example 2

[0063] Add oxidized cellulose to the eutectic solvent and dissolve it at 50 °C, and the other conditions are the same as in Example 1.

[0064] Comparative Example 3

[0065] Add 3 g of oxidized cellulose (absolute dry weight) to 30 g of the eutectic solvent, and the other conditions are the same as in Example 1.

[0066] Comparative Example 4

[0067] Sodium periodate was not added, and the other conditions were the same as in Example 1.

[0068] Comparative Example 5

[0069] Urea was used to replace resorcinol, and the other conditions were the same as those in Example 1.

[0070] The dissolution states of cellulose in Examples 1 to 5 and Comparative Examples 1 to 5 were observed under a microscope. From the results, it was found that the cellulose in Examples 1 to 5 was completely dissolved, all being transparent cellulose solutions; in Comparative Examples 1 to 3, the cellulose was partially dissolved, and in Comparative Examples 4 to 5, the amount of cellulose dissolved was very small and it was difficult to dissolve.

[0071] Figure 4 Figures showing the changes in the actual objects of cellulose dissolution in Examples 1 to 5 and Comparative Examples 1 and 4. Among them, MCC in the figures is cellulose, and DAC is dialdehyde cellulose (oxidized cellulose) obtained after cellulose oxidation. DAC 0.6 / 5% in Example 1 means that the oxidized cellulose accounts for 5% of the mass of the deep eutectic solvent, and the mass ratio of sodium periodate to microcrystalline cellulose is 0.6:1; DAC 0.4 / 5% in Example 2 means that the oxidized cellulose accounts for 5% of the mass of the deep eutectic solvent, and the mass ratio of sodium periodate to microcrystalline cellulose is 0.4:1; DAC 0.6 / 15% in Example 3 means that the oxidized cellulose accounts for 15% of the mass of the deep eutectic solvent, and the mass ratio of potassium periodate to α-cellulose is 0.6:1; DAC 0.8 / 20% in Example 4 means that the oxidized cellulose accounts for 20% of the mass of the deep eutectic solvent, and the mass ratio of the oxidant to the cellulose extracted from herbaceous plants is 0.8:1; DAC 0.8 / 5% in Example 5 means that the oxidized cellulose accounts for 5% of the mass of the deep eutectic solvent, and the mass ratio of ammonium cerium nitrate to microcrystalline cellulose is 0.8:1; DAC 0.3 / 5% in Comparative Example 1 means that the oxidized cellulose accounts for 5% of the mass of the deep eutectic solvent, and the mass ratio of sodium periodate to microcrystalline cellulose is 0.3:1; MCC-5% in Comparative Example 4 means that microcrystalline cellulose accounts for 5% of the mass of the deep eutectic solvent, and the microcrystalline cellulose was not oxidized.

[0072] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for dissolving cellulose with a deep eutectic solvent, characterized in that, Specifically, the steps are as follows: 1) Mix cellulose, an oxidizing agent, and water, and then carry out an oxidation reaction to obtain oxidized cellulose: 2) Mix the oxidized cellulose with a deep eutectic solvent to obtain a transparent cellulose solution; The cellulose in step 1) includes one or more of cellulose extracted from plants, microcrystalline cellulose, and α-cellulose; the oxidizing agent is periodate and / or ammonium cerium nitrate, and the periodate is sodium periodate or potassium periodate; In step 1), the mass ratio of the cellulose, the oxidizing agent, and water is 1:0.4 - 1:5 - 20, the temperature of the oxidation reaction is 25 - 75°C, and the time is 4 - 24 h; In step 2), the mass ratio of the oxidized cellulose to the deep eutectic solvent is 0.05 - 0.35:1; The deep eutectic solvent includes a hydrogen bond donor and a hydrogen bond acceptor; the molar ratio of the hydrogen bond donor to the hydrogen bond acceptor is 1:0.5 - 3: The hydrogen bond donor is one or more of resorcinol, hydroquinone, cresol, thymol, and bisphenol A; The hydrogen bond acceptor is one or more of tetramethylammonium chloride, betaine, and methyltriphenylphosphonium bromide; the deep eutectic solvent is obtained by mixing the hydrogen bond donor and the hydrogen bond acceptor; the temperature of the mixing is 60 - 100°C, and the time is 0.5 - 2 h.

2. The method according to claim 1, wherein The plant is a herbaceous plant or a woody plant.

3. The method according to claim 1, wherein The oxidation rate of the oxidized cellulose is 5 - 50%.

4. The method according to claim 1, characterized in that, In step 2), the temperature of the mixing is 60 - 120°C, and the time is 0.5 - 4 h; the mixing is carried out under the conditions of stirring, ultrasonic treatment, internal mixing, open mixing, or extrusion processing.

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