A method for preparing nanocellulose based on ion liquid / water system dissociation of biomass

Abstract

The application belongs to the field of comprehensive utilization of biomass, and relates to a method for preparing nanocellulose based on dissociation of biomass in an ionic liquid / water system. Cellulose fibers are added to a binary system composed of ionic liquid and water, and are heated and stirred under high pressure to obtain a cellulose dispersion liquid, which is regenerated and dried to obtain nanocellulose. Compared with the prior art, the process provided by the application is simpler, greener and more efficient. The raw material used in the application belongs to renewable resources, and the nanocellulose prepared therefrom can be widely applied in medicine, fine chemical industry and preparation of new materials, thereby providing a new method for efficient utilization of renewable resources.

Description

Technical Field

[0001] This invention belongs to the field of comprehensive utilization of biomass and relates to a method for preparing modified cellulose, namely nanocellulose, and particularly to a method for preparing nanocellulose based on the dissociation of biomass in an ionic liquid / water system. Background Technology

[0002] Cellulose is a renewable, biodegradable, and non-toxic material. It is also a significant source of environmentally friendly and biocompatible products. It is estimated that 10% of cellulose is produced through photosynthesis. 11 ~10 12 Tons per year. The main sources of cellulose are wood and cotton. These materials have been used in the textile and paper industries for centuries. Furthermore, modification of cellulose can yield products with various functions. Recently, interest in nanocellulose has increased dramatically. Nanocellulose is a nanoscale material extracted from cellulose, typically less than 100 nanometers in diameter, with lengths ranging from hundreds of nanometers to several micrometers. Nanocellulose exhibits excellent application potential in various fields such as medicine, fine chemicals, and the preparation of novel materials due to its superior mechanical properties, dimensional stability, biodegradability, and impact resistance.

[0003] Cellulose nanofibers can be extracted from protocellulose through various methods, including biological methods (enzymatic hydrolysis), mechanical methods (such as grinding, ultrasonication, and high-pressure homogenization), and chemical methods (acid hydrolysis). Enzymatic hydrolysis selectively modifies and digests fibers to produce high-strength nanofibers, but it is costly. Acid hydrolysis can obtain cellulose nanofibers (CNFs) with uniform particle size and morphology, but the strong acids used may pollute the environment. Mechanical methods break down cellulose raw materials into nanoscale fibers through physical means. Ultrasonication and high-pressure homogenization are commonly used mechanical methods that can produce CNFs with uniform size and improve homogenization efficiency. However, they are energy-intensive, have high equipment costs, are prone to clogging, and are difficult to operate continuously. Cryogenic crushing technology combined with liquid nitrogen and chemical treatment can improve the thermal properties of nanofibers. Ball milling breaks fibers into nanoscale particles through physical shear force. It is simple to operate and has relatively low equipment costs. However, the CNFs produced are not uniform in size, and may require multiple grindings to achieve the desired size, resulting in low efficiency and high energy consumption. Electrospinning involves dissolving cellulose in a specific solvent and then spraying it out under an electric field to form nanofibers. This method can produce CNFs with small and uniform diameters. However, it requires suitable solvents, may involve chemical reagents, has high equipment costs, and the fiber collection and processing are relatively complex.

[0004] Chinese patent CN 106146877 A discloses a method for recycling waste textiles using ionic liquids. The method involves pre-treating waste textiles, mixing them with ionic liquid and water, and stirring under vacuum to obtain a cellulose-containing liquid. After centrifugation, a cellulose solution is obtained, which is then degassed, coagulated, and regenerated to yield cellulose fibers. However, the cotton fiber recovery rate after regeneration from the dissolved cellulose solution is low, only 60%, and the method of dissolving textiles for regeneration makes it difficult to obtain nanofibers. Chinese patent CN102505546A discloses a homogeneous method for preparing nanocellulose. This method involves adding ionic liquid to pulverized wood fibers, microwaving and stirring, followed by high-pressure homogenization. The resulting solution is regenerated and dried to obtain nanocellulose. However, this method requires alkali treatment of the wood fibers, microwave heating, and high-pressure shearing using a homogenizer. The equipment is costly, energy-intensive, and inefficient; the homogenizer is prone to clogging, making continuous operation difficult. Therefore, developing a novel, simple, green, and efficient method for preparing nanocellulose is essential. Summary of the Invention

[0005] The technical problem this invention aims to solve is to address the shortcomings of existing technologies by providing a novel one-step method for preparing nanocellulose. This invention uses an ionic liquid and water binary system as a solvent to break the hydrogen bonds between cellulose matrix fibers, allowing cellulose to controllably dissociate into nanocellulose without dissolving in solution. This process is simple, energy-efficient, and environmentally friendly, providing a new approach for the preparation of nano-sized cellulose.

[0006] Invention concept: By controlling the hydrogen bond alkalinity of the ionic liquid / water system, the hydrogen bond connections between cellulose microfibrils and between basic filaments are broken, while the hydrogen bonds between the cellulose chains inside the cellulose microfibrils and basic filaments are preserved, so as to realize the transformation of cellulose into cellulose nanofibers without dissolving into single chains.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] This invention discloses a method for preparing nanocellulose based on the dissociation of biomass using an ionic liquid / water system. Cellulose fibers are added to a binary system composed of an ionic liquid and water, and the mixture is heated and stirred under high pressure of 0.3 MPa to 30 MPa to dissociate the cellulose. The resulting cellulose dispersion is then regenerated and dried to obtain nanocellulose.

[0009] In some embodiments, the cellulose fiber is any one or a combination of two of cotton and wood pulp.

[0010] The cellulose fibers mentioned herein need to be washed and dried before use; if the cellulose fibers mentioned are waste cellulose fibers, they need to be disinfected, decolorized, washed, and dried before use.

[0011] In some embodiments, the cation of the ionic liquid is any one of substituted or unsubstituted quaternary ammonium cations, imidazole cations, and pyridine cations.

[0012] In some embodiments, the cation of the ionic liquid is substituted with one or more C1-C12 alkyl groups and / or substituted with one or more propylene groups.

[0013] In some embodiments, preferably, the cation of the ionic liquid is substituted with one or more C1 alkyl groups, and / or with one or more C2 alkyl groups, and / or with one or more C4 alkyl groups, and / or with one or more C12 alkyl groups, and / or with one or more propenyl groups.

[0014] In some embodiments, the anion of the ionic liquid is a halide anion, a carboxylate anion, or BF4. - PF6 - SCN - and CN - Any one of them.

[0015] In some embodiments, preferably, the anion of the ionic liquid is a halide anion or a carboxylate anion.

[0016] In some embodiments, preferably, the ionic liquid is N,N,N-3-methyl-N-dodecyl ammonium acetate ionic liquid, 1-butyl-3-methylimidazolium chloride ionic liquid, 1-propenyl-3-methylimidazolium chloride ionic liquid or 1-ethylpyridine chloride ionic liquid.

[0017] In some embodiments, in the binary system composed of ionic liquid and water, the mass ratio of ionic liquid to water is 98:2 to 50:50.

[0018] In some embodiments, preferably, in the binary system composed of ionic liquid and water, the mass ratio of ionic liquid to water is 98:2 to 95:5.

[0019] In some embodiments, the mass ratio of the cellulose fiber to the binary system composed of the ionic liquid and water is 1:10 to 1:100.

[0020] In some embodiments, the mass ratio of the cellulose fiber to the binary system composed of the ionic liquid and water is 1:50 to 1:100.

[0021] In some embodiments, the dissociation occurs at a temperature of 30°C to 160°C and for a time of 1 hour to 24 hours; the dissociation is carried out under an inert gas atmosphere.

[0022] In some embodiments, preferably, the dissociation occurs at a temperature of 30°C to 120°C, and more preferably at 30°C to 105°C.

[0023] The inert gas is preferably nitrogen.

[0024] In some embodiments, during the regeneration process, a polar solvent is added to the cellulose dispersion to cause cellulose to precipitate.

[0025] In some embodiments, the polar solvent is any one or a combination of several of water, ethanol and acetone; the volume ratio of the cellulose dispersion to the polar solvent is 1:4 to 1:8.

[0026] In some embodiments, preferably, the polar solvent is ethanol.

[0027] In some embodiments, the drying is vacuum freeze drying or spray drying.

[0028] Beneficial effects:

[0029] (1) The raw materials used in this invention are renewable resources. When they are prepared into nanocellulose, they can be widely used in medicine, fine chemicals, and the preparation of new materials, providing a new method for the efficient utilization of renewable resources.

[0030] (2) The method for preparing nanocellulose provided by the present invention can obtain nanocellulose with high yield and small diameter by breaking the hydrogen bonds between cellulose microfibrils and between basic fibrils. The yield is higher than 65%, and the minimum diameter of nanocellulose can be as small as 3 nm.

[0031] (3) The present invention uses ionic liquid as solvent, which is non-toxic and non-volatile in the production process and is easy to recycle, thus achieving the goal of clean and green production of cellulose fibers.

[0032] (4) This invention achieves the preparation of high-performance cellulose nanofibers in one step by regulating the hydrogen bonding effect of ionic liquid and water binary system on cellulose, without acid, pretreatment, or mechanical fibrillation.

[0033] (5) Compared with the prior art (Chinese patent CN102505546A and Chinese patent CN106146877A), the process provided by the present invention has two outstanding advantages: ① The process provided by the present invention does not use a high-pressure homogenizer, so there is no problem of equipment blockage and continuous production is possible; ② The process provided by the present invention does not dissolve cellulose, so it will not destroy the crystal structure of cellulose and has a high degree of crystallinity, which can reach 70%.

[0034] (6) Compared with the prior art, the process provided by the present invention is simpler, greener and more efficient. Attached Figure Description

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, and the advantages of the present invention in the above and / or other aspects will become clearer.

[0036] Figure 1 These are the infrared spectra of the nanocellulose prepared in Examples 3 and 4.

[0037] Figure 2 This is a transmission electron microscope image of the nanocellulose prepared in Example 4.

[0038] Figure 3 Transmission electron microscope image of the regenerated cellulose prepared for Comparative Example 1. Detailed Implementation

[0039] The present invention can be better understood from the following embodiments. However, those skilled in the art will readily understand that the descriptions in the embodiments are for illustrative purposes only and should not, and will not, limit the invention as detailed in the claims.

[0040] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are commercially available.

[0041] The ethanol used in the following embodiments of the present invention is anhydrous ethanol.

[0042] The ionic liquids used in the embodiments of this invention can be commercially available or prepared according to existing technology (N,N,N-3-methyl-N-dodecyl ammonium acetate, CAS122151-02-4; 1-butyl-3-methylimidazolium chloride, CAS 79917-90-1; 1-ethylpyridine chloride ionic liquid, CAS2294-38-4; 1-propenyl-3-methylimidazolium chloride ionic liquid, CAS 1331838-15-3).

[0043] Example 1:

[0044] A binary system of N,N,N-3-methyl-N-dodecyl acetate ammonium and water was prepared, with water comprising 50% by mass. Washed and dried cotton was added to the binary system of N,N,N-3-methyl-N-dodecyl acetate ammonium and water, and the mixture was placed in a high-pressure reactor with a cotton-to-binary system mass ratio of 1:10. Subsequently, the reactor was pressurized to 30 MPa under a nitrogen atmosphere and heated and stirred at 160 °C for 24 h. After heating and stirring, ethanol was added to the resulting dispersion while stirring to precipitate cellulose. The volume ratio of the dispersion to ethanol was 1:4. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0045] The nanocellulose obtained in this embodiment has an average diameter of 90 nm, a minimum diameter of 3 nm, a yield of 79.36%, and a crystallinity of 70%.

[0046] Example 2:

[0047] A binary system of 1-butyl-3-methylimidazolium chloride ion liquid and water was prepared, with the water mass fraction being 3%. Washed and dried cotton was added to the binary system of 1-butyl-3-methylimidazolium chloride ion liquid and water, and the mixture was placed in a high-pressure reactor with a cotton-to-binary system mass ratio of 1:100. Subsequently, the reactor was pressurized to 10 MPa under a N2 atmosphere and heated and stirred at 30°C for 1 h. After heating and stirring, ethanol was added to the resulting mixed dispersion while stirring to precipitate cellulose. The volume ratio of the mixed dispersion to ethanol was 1:8. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0048] The nanocellulose obtained in this embodiment has an average diameter of 90 nm, a minimum diameter of 5 nm, a yield of 78.91%, and a crystallinity of 78%.

[0049] Example 3:

[0050] A binary system of 1-propenyl-3-methylimidazolium chloride ion liquid and water was prepared, with water comprising 5% by mass. Washed and dried cotton was added to the binary system of 1-propenyl-3-methylimidazolium chloride ion liquid and water, and the mixture was placed in a high-pressure reactor with a cotton-to-binary system mass ratio of 1:50. Subsequently, the reactor was pressurized to 0.3 MPa under a nitrogen atmosphere and heated and stirred at 105 °C for 3 h. After heating and stirring, ethanol was added to the resulting mixed dispersion while stirring to precipitate cellulose. The volume ratio of the mixed dispersion to ethanol was 1:4. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0051] The nanocellulose obtained in this example has an average diameter of 60 nm, a minimum diameter of 4 nm, a yield of 87.23%, and a crystallinity of 74%.

[0052] Example 4:

[0053] A binary system of 1-ethylpyridine chloride ion liquid and water was prepared, with the water mass fraction being 2%. Washed and dried cotton was added to the binary system of 1-ethylpyridine chloride ion liquid and water, and the mixture was placed in a high-pressure reactor with a cotton-to-binary system mass ratio of 1:50. Subsequently, the reactor was pressurized to 4 MPa under a N2 atmosphere and heated and stirred at 105 °C for 3 h. After heating and stirring, ethanol was added to the resulting mixed dispersion while stirring to precipitate cellulose. The volume ratio of the mixed dispersion to ethanol was 1:4. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0054] The nanocellulose obtained in this embodiment has an average diameter of 60 nm, a minimum diameter of 5 nm, a yield of 85.78%, and a crystallinity of 76%.

[0055] The infrared spectra of the nanocellulose prepared in Examples 3 and 4 are shown below. Figure 1 As shown, CNF (5% 105℃ 3h) is the nanocellulose prepared in Example 3, CNF (2% 105℃ 3h) is the nanocellulose prepared in Example 4, and cotton is the raw material cotton. Infrared spectroscopy analysis results show that the infrared signal peak positions of the nanocellulose and the raw cotton are basically the same, and the peak shapes are basically consistent, indicating that no new functional groups are formed. This suggests that no derivatization reaction occurred during the entire dissociation process, which to some extent indicates that the process of this application does not significantly change the biocompatibility of the fiber itself, i.e., this process has good safety.

[0056] Transmission electron microscope image of the nanocellulose prepared in Example 4 is shown below. Figure 2 As shown, this method can reduce the average fiber diameter to about 60 nm.

[0057] Comparative Example 1: Recycling Waste Textiles Using Ionic Liquids

[0058] The experimental method used in this comparative example is the same as that used in Example 1 of Chinese Patent CN106146877A, as detailed below:

[0059] (1) Water swelling and ionic liquid dissolution: The cotton was dissolved in an ionic liquid with the following material ratio: 2.002 g of cotton, 98.041 g of ionic liquid [Amim]Cl and 2.023 g of water (i.e., mass ratio of 2:98:2). The mixture was stirred and dissolved for 1 h at 70 °C and under vacuum (140 Pa). During the dissolution process, the cotton first absorbed water and swelled, and then dissolved in the ionic liquid to obtain a clear solution, i.e., a cellulose solution. Under a polarizing microscope, the solution was black throughout the field of view, indicating that the cotton was completely dissolved.

[0060] (2) Cellulose regeneration: Ethanol was added to the cellulose solution obtained in step (1) to regenerate cellulose. The regenerated cellulose was washed with deionized water and then freeze-dried to obtain regenerated cellulose. TEM observation showed no nanocellulose and that it was a blocky solid, as shown in the following figure. Figure 3 As shown.

[0061] Example 5:

[0062] A binary system of N,N,N-3-methyl-N-dodecyl acetate ammonium and water was prepared, with water comprising 50% by mass. Washed and dried wood pulp was added to the binary system of N,N,N-3-methyl-N-dodecyl acetate ammonium and water, and the mixture was placed in a high-pressure reactor with a wood pulp to binary system mass ratio of 1:10. Subsequently, the reactor was pressurized to 30 MPa under a nitrogen atmosphere and heated and stirred at 160 °C for 24 h. After heating and stirring, ethanol was added to the resulting mixed dispersion while stirring to precipitate cellulose. The volume ratio of the mixed dispersion to ethanol was 1:4. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0063] The nanocellulose obtained in this embodiment has a diameter of 70 nm, a minimum diameter of 4 nm, a yield of 68.76%, and a crystallinity of 71%.

[0064] Example 6:

[0065] A binary system of 1-butyl-3-methylimidazolium chloride ion liquid and water was prepared, with the water mass fraction being 3%. Washed and dried wood pulp was added to the binary system of 1-butyl-3-methylimidazolium chloride ion liquid and water, and the mixture was placed in a high-pressure reactor with a wood pulp to binary system mass ratio of 1:100. Subsequently, the reactor was pressurized to 10 MPa under a N2 atmosphere and heated and stirred at 30°C for 1 h. After heating and stirring, ethanol was added to the resulting mixed dispersion while stirring to precipitate cellulose. The volume ratio of the mixed dispersion to ethanol was 1:8. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0066] The nanocellulose obtained in this embodiment has a diameter of 80 nm, a minimum diameter of 5 nm, a yield of 82.31%, and a crystallinity of 79%.

[0067] Example 7:

[0068] A binary system of 1-propenyl-3-methylimidazolium chloride ion liquid and water was prepared, with water comprising 5% by mass. Washed and dried wood pulp was added to the binary system of 1-propenyl-3-methylimidazolium chloride ion liquid and water, and the mixture was placed in a high-pressure reactor with a wood pulp to binary system mass ratio of 1:50. Subsequently, the reactor was pressurized to 0.3 MPa under a N2 atmosphere and heated and stirred at 105 °C for 3 h. After heating and stirring, ethanol was added to the resulting mixed dispersion while stirring to precipitate cellulose, with a mixed dispersion to ethanol volume ratio of 1:4. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0069] The nanocellulose obtained in this embodiment has a diameter of 50 nm, a minimum diameter of 4 nm, a yield of 88.34%, and a crystallinity of 73%.

[0070] Example 8:

[0071] A binary system of 1-ethylpyridine chloride ion liquid and water was prepared, with the water mass fraction being 2%. Washed and dried wood pulp was added to the binary system of 1-ethylpyridine chloride ion liquid and water, and the mixture was placed in a high-pressure reactor with a wood pulp to binary system mass ratio of 1:50. Subsequently, the reactor was pressurized to 4 MPa under a N2 atmosphere and heated and stirred at 105 °C for 3 h. After heating and stirring, ethanol was added to the resulting mixed dispersion while stirring to precipitate cellulose. The volume ratio of the mixed dispersion to ethanol was 1:4. The precipitate was centrifuged, washed several times with ethanol, and freeze-dried to obtain nanocellulose.

[0072] The nanocellulose obtained in this example has an average diameter of 45 nm, a minimum diameter of 3 nm, a yield of 82.75%, and a crystallinity of 72%.

[0073] This invention provides a method for preparing nanocellulose based on the dissociation of biomass using an ionic liquid / water system. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A method for preparing nanocellulose based on the dissociation of biomass using an ionic liquid / water system, characterized in that, Cellulose fibers are added to a binary system consisting of an ionic liquid and water, and then heated and stirred under high pressure of 0.3 MPa to 30 MPa to dissociate the cellulose. The resulting cellulose dispersion is then regenerated and dried to obtain nanocellulose.

2. The method according to claim 1, characterized in that, The cellulose fiber is any one or a combination of two of cotton and wood pulp.

3. The method according to claim 1, characterized in that, The cation of the ionic liquid is any one of substituted or unsubstituted quaternary ammonium cations, imidazole cations, and pyridine cations.

4. The method according to claim 1, characterized in that, The cation of the ionic liquid is substituted with one or more C1-C12 alkyl groups and / or substituted with one or more propenyl groups.

5. The method according to claim 1, characterized in that, The anions of the ionic liquid are halide anions, carboxylate anions, and BF4. - PF6 - SCN - and CN - Any one of them.

6. The method according to claim 1, characterized in that, In the binary system composed of the ionic liquid and water, the mass ratio of the ionic liquid to water is 98:2 to 50:

50.

7. The method according to claim 1, characterized in that, The mass ratio of the cellulose fiber to the binary system composed of the ionic liquid and water is 1:10 to 1:

100.

8. The method according to claim 1, characterized in that, The dissociation is carried out at a temperature of 30℃ to 160℃ and for a time of 1h to 24h, and is conducted under an inert gas atmosphere.

9. The method according to claim 1, characterized in that, During the regeneration process, a polar solvent is added to the cellulose dispersion to cause cellulose to precipitate.

10. The method according to claim 9, characterized in that, The polar solvent is any one or a combination of several of water, ethanol and acetone; the volume ratio of the cellulose dispersion to the polar solvent is 1:4 to 1:8.

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