Preparation method of controllable modified nanometer lignocellulose

Abstract

This invention belongs to the field of high-value utilization technology of biomass resources, specifically relating to a method for preparing controllably modified lignocellulose nanofibers. After fully swelling lignocellulose in a eutectic solvent, acid anhydrides and organic acid esterification modifiers are added to flexibly functionalize the swollen lignocellulose, preparing controllably modified lignocellulose nanofibers. This invention fully swells lignocellulose raw materials in a eutectic solvent system, breaking the stubborn hydrogen bond network while retaining the surface active hydroxyl groups of lignocellulose and improving its accessibility. The eutectic solvent effectively lowers the melting point of individual components, thus allowing compatibility with various active acid anhydrides and organic acid esterification modifiers with high melting points and poor solubility, thereby achieving mild and efficient functional modification of lignocellulose. This method can controllably prepare lignocellulose nanofibers containing carboxyl groups, double bonds, benzene rings, alkyl groups, acetyl groups, lactyl groups, and bridged methylene groups.

Description

Technical Field

[0001] This invention belongs to the field of high-value utilization technology of biomass resources, and specifically relates to a method for preparing controllable modified nano-lignocellulose. Background Technology

[0002] Lignocellulose is the most abundant renewable biomass polymer on land, widely found in agricultural and forestry resources. Nano-lignincellulose can be prepared and functionalized through chemical pretreatment and mechanical processing of plant materials. As a renewable biomass nanofiber, nano-lignincellulose serves as a fundamental nanomaterial and has been widely used in composite materials, lignincellulose-based functional materials, and renewable energy in recent years. However, as a versatile fundamental nanomaterial, the preparation of nano-lignincellulose is often limited by the swelling strength of the solvent system, the reactivity of the modifier, the recoverability and cost of the liquid system, and the energy consumption and efficiency of mechanical processing. This makes it difficult to achieve large-scale production and processing of this fundamental nanomaterial, and hinders low-energy, flexible, and controllable functionalization modification. This significantly limits the potential for high-value applications of nano-lignincellulose.

[0003] Eutectic solvents are a novel type of green solvent system composed of hydrogen bond donors and acceptors. The most significant characteristics of eutectic solvents include low cost, flexible formulation, good biocompatibility, and high thermal and chemical stability. In recent years, many researchers have discovered that the abundant hydrogen bond network in eutectic solvents is beneficial for effectively disrupting the stubborn hydrogen bond structure in lignocellulose raw materials through hydrogen bond competition, thus achieving sufficient swelling. Sufficiently swollen lignocellulose effectively enhances the accessibility of its surface hydroxyl groups, enabling efficient preparation and functional modification of nano-lignocellulose. It is considered the most suitable solvent system for cellulose nanofiberization and has the potential for large-scale and scalable production. CN114277461A describes a eutectic solvent system composed of citric acid, choline chloride, and water to prepare carboxylated nanocellulose through esterification modification of cellulose raw materials after pretreatment. Although this method is convenient and produces carboxylated cellulose nanofibers with a mild reaction, it is limited by the swelling capacity of the citric acid / choline chloride system and the esterification activity of citric acid, resulting in only a single type of esterification modification of citric acid being possible under this system. Furthermore, this method only applies to cellulose and does not address lignin-containing woody fibers. CN115368585A proposes a eutectic solvent system composed of choline chloride and lactic acid for pretreatment of bagasse raw materials, followed by carboxylation modification via succinic anhydride. The cellulose nanofibers produced by this method have advantages such as narrow diameter distribution, uniform size, and are green, non-toxic, biodegradable, and renewable. However, the inevitable lactic acid / choline chloride esterification modification during the swelling process is a competing reaction with the succinic anhydride esterification modification. The compatibility of various modifiers with the eutectic solvent is also limited by the compatibility of lactic acid / choline chloride with the modifier, making it difficult to achieve efficient modification of multiple functional groups under control. Therefore, achieving high aspect ratios and controllable modification of multifunctional groups in eutectic solvent systems for nano-lignocellulose remains constrained by the swelling capacity of the eutectic solvent, the reactivity of the modifier, and the compatibility between the eutectic solvent and the modifier. This makes it difficult to achieve controllable modification of multifunctional groups in nano-lignocellulose as a fundamental nanomaterial, thus hindering its application in various materials science fields.

[0004] The efficient swelling capacity of eutectic solvent systems for lignocellulose is often overlooked in the production of controllably modified nano-lignocellulose. Lignocellulose raw materials swell efficiently in eutectic solvents, while disrupting their stubborn natural structure, retain their crystalline structure and structural strength. This process breaks down the rigid hydrogen bond network while preserving the surface-active hydroxyl groups of lignocellulose and improving their accessibility. This is crucial for subsequent controllable functionalization and mechanical dispersion processes. With sufficient swelling, the surface-active hydroxyl groups of lignocellulose are fully exposed, allowing for flexible selection of modifiers to achieve controllable modification of various functional groups using this method. Simultaneously, the significant reduction in the melting point of individual components by eutectic solvents lowers the melting point of various active modifiers and enables them to exhibit excellent compatibility with eutectic solvents. This allows for the efficient functionalization of lignocellulose by various active esterification modifiers with high melting points and poor solubility, thus satisfying the need for flexible and controllable preparation of modified nano-lignocellulose under mild conditions. Ultimately, this meets the requirements of complex applications and allows for the production of customized nano-lignocellulose with specific microstructures and surface functional groups.

[0005] Therefore, this invention is proposed. Summary of the Invention

[0006] Purpose of the Invention: Traditional eutectic solvent systems for preparing functionalized modified nano-lignocellulose offer diverse options and flexible combinations, but all eutectic solvent systems play a dual role in swelling and modification. While simultaneous swelling and modification of lignocellulose within the same eutectic solvent system offers advantages such as ease of operation and flexible control, it also limits the types of modifications, controllability, reaction efficiency, and solvent recovery efficiency under this strategy. Furthermore, the swelling and modification capabilities of lignocellulose within the same eutectic solvent system are often mismatched, with different emphases on the two. This results in many eutectic solvent systems exhibiting significant swelling capacity but low modification efficiency; conversely, some eutectic solvent systems have high modification efficiency but insufficient swelling capacity, making it difficult to produce nano-lignocellulose with high aspect ratios and various functional groups. This forces current flexible and controllable modified nano-lignocellulose to attempt to solve this problem through the extensive and cumbersome development of novel eutectic solvent systems, which is not only inefficient but also limits the precise control of the swelling and modification behavior of lignocellulose by eutectic solvent systems.

[0007] To address the above problems, this invention provides a method for preparing controllably modified nano-lignocellulose. The aim is to utilize a eutectic solvent to efficiently swell lignocellulose raw materials, first disrupting their stubborn natural structure while maintaining their crystal structure and structural strength. This process breaks down the stubborn hydrogen bond network while preserving the surface-active hydroxyl groups of lignocellulose and improving their accessibility. Subsequently, based on the fully swollen lignocellulose, active anhydrides or organic acids are introduced as modifiers, thereby enabling flexible and controllable modification of various functional groups using this method. Notably, the eutectic solvent's significant reduction in the melting point of individual components endows numerous modifiers with excellent compatibility, allowing various active esterification modifiers with high melting points and poor solubility to efficiently perform functional modification of lignocellulose at lower temperatures. This satisfies the need for flexible and controllable preparation of modified nano-lignocellulose under mild conditions.

[0008] Technical solution: To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A method for preparing controllably modified nano-lignocellulose, characterized by comprising the following steps: S1. The lignocellulose raw material is swollen using a eutectic solvent, wherein the content of the lignocellulose raw material in the eutectic solvent is 0.1-20 wt%, and excess eutectic solvent is separated for reuse. The eutectic solvent is composed of a hydrogen bond donor, a hydrogen bond acceptor, and water. The hydrogen bond donor is any one or a combination of two or more of zinc acetate, zinc chloride, aluminum chloride, and lithium chloride, preferably zinc acetate and / or zinc chloride. The hydrogen bond acceptor is any one or a combination of two or more of choline chloride, choline bromide, choline acetate, tetramethylammonium bromide, tetrabutylammonium bromide, benzyltrimethylammonium chloride, or betaine, preferably any one or a combination of two or more of choline chloride, choline bromide, tetramethylammonium bromide, tetrabutylammonium bromide, or betaine. 0-50 wt% water is added to the eutectic solvent as a hydrogen bond modifier, preferably 0-20 wt%.

[0009] S2. Add an esterification modifier to the fully swollen lignocellulose / eutectic solvent mixture obtained in step S1 for mild and controllable esterification modification, separate excess esterification modifier for reuse, and then prepare a surface-esterified nano-lignocellulose dispersion through mechanical processing; the modifier is an acid anhydride and / or an organic acid; the mechanical processing is one or more of the following mechanical dispersion treatments: cell wall breaking machine, colloid mill, ultrasonication, ultrafine grinding, high pressure homogenization.

[0010] Furthermore, the lignocellulose raw materials are cellulose, wood, grass, bamboo, and hemp.

[0011] Furthermore, the acid anhydride or organic acid esterification modifier is any one or a combination of two or more of maleic anhydride, succinic anhydride, phthalic anhydride, norborneol enediic anhydride, dodecenylsuccinic anhydride, octenylsuccinic anhydride, acetic anhydride, propionic anhydride, lactic acid, formic acid, acetic acid, oxalic acid, maleic acid, and citric acid, preferably any one or a combination of two or more of maleic anhydride, succinic anhydride, phthalic anhydride, norborneol enediic anhydride, dodecenylsuccinic anhydride, acetic anhydride, lactic acid, oxalic acid, maleic acid, and citric acid.

[0012] Furthermore, after swelling, excess eutectic solvent is separated to adjust the concentration of the eutectic solvent dispersion of fully swollen lignocellulose to 1-20 wt%.

[0013] Furthermore, the separated eutectic solvent can be quickly separated through a filter and recycled, improving the process flow and reducing separation costs.

[0014] Furthermore, the concentration of eutectic solvent-swelled lignocellulose is 0.1-50 wt%, preferably 0.1-20 wt%.

[0015] Furthermore, the reaction temperature for eutectic solvent swelling of lignocellulose is 50-120°C. o C, preferably 60-95 o C.

[0016] Furthermore, the reaction time for swelling lignocellulose in the eutectic solvent is 30-240 min, preferably 30-180 min.

[0017] Furthermore, the fully swollen lignocellulose accounts for 0.1-10 wt% of the total mass of the eutectic solvent and esterification modifier.

[0018] Furthermore, the modification temperature for fully swollen lignocellulose is 60-120°C. o C, preferably 60-95 o C Furthermore, the modification time for fully swollen lignocellulose is 5-180 min, preferably 5-120 min.

[0019] Furthermore, the surface of the anhydride-modified nano-lignocellulose contains active carboxyl groups and functional groups in the same molar ratio, wherein the content of the active carboxyl groups and functional groups is 0.5-2.5 mmol / g, preferably 0.8-2.1 mmol / g.

[0020] Furthermore, the surface of the nano-lignocellulose modified with organic acid contains active carboxyl groups, the content of which is 1.0-2.5 mmol / g, preferably 1.0-2.0 mmol / g.

[0021] Furthermore, the controllable modified nano-lignocellulose has a diameter of 3-20 nm, an aspect ratio of 500-3000, and a yield of 80-93%.

[0022] Furthermore, the controllable modified nano-lignocellulose has functionalized modifying groups that are any one or more combinations of carboxyl, double bond, benzene ring, alkyl, acetyl, lactyl, and bridged methylene groups.

[0023] Furthermore, after the swollen lignocellulose is controlled and modified with a modifier, the residual eutectic solvent and modifier can be quickly separated through a filter screen and reused.

[0024] Furthermore, the modified lignocellulose, after rapid separation through a filter, reduces chemical consumption during the washing process, and mechanical treatment under central conditions solves the problem of equipment corrosion under acidic conditions.

[0025] According to another aspect of the present invention, controllable modification of nano-lignocellulose in a eutectic solvent system can be used in the fields of functionalized membrane materials, gel / elastomer materials, composite materials, bioplastics and nanofillers.

[0026] Gain Effect: Compared with existing nano-lignin cellulose preparation technologies, the advantages of this invention include: 1) This invention adopts a strategy of swelling before modification in a eutectic solvent system, which gives full play to the swelling effect of the eutectic solvent on lignocellulose raw materials. While destroying the stubborn natural structure, it can maintain the crystal structure and structural strength. While breaking the stubborn hydrogen bond network, it retains the surface active hydroxyl groups of lignocellulose and improves its accessibility. This will help improve the efficiency and reactivity of subsequent controllable functionalization modification.

[0027] 2) The eutectic solvent system used in this invention can fully swell nano-lignocellulose without additional chemical modification of lignocellulose. It only gives full play to the eutectic solvent's characteristic of full swelling, which effectively solves the drawback of mismatch between swelling and modification capabilities in traditional eutectic solvent methods for preparing nano-lignocellulose. This allows for precise control of the swelling and modification behavior of lignocellulose by the eutectic solvent system.

[0028] 3) This invention utilizes the characteristic that eutectic solvents can significantly reduce the melting point of single components, thereby lowering the melting point of various active modifiers and exhibiting excellent compatibility with eutectic solvents. This allows for the efficient functional modification of lignocellulose by various active esterification modifiers with high melting points and poor solubility, thus satisfying the need for flexible and controllable preparation of modified nano-lignocellulose under mild preparation conditions.

[0029] 4) In this invention, the residual solvent system can be efficiently recovered and recycled by using a filter screen after swelling and modification of the eutectic solvent, which greatly improves the process cost and operation difficulty in the preparation of nano-lignocellulose and has the potential for large-scale production.

[0030] 5) Based on the lignocellulose raw material that has been fully swollen by the eutectic solvent, the type and degree of modification of nano-lignocellulose can be flexibly controlled by flexibly selecting the type of modifier, the concentration of lignocellulose, the reaction parameters, and the ratio of eutectic solvent and modifier, so as to meet the complex requirements of nano-cellulose in complex application scenarios. Attached Figure Description

[0031] Figure 1 These are scanning electron microscope images of lignocellulose raw materials after they have been fully swollen by a eutectic solvent. Figure 2 These are scanning electron microscope images and particle size distributions of nano-lignocellulose that have been fully swollen in a eutectic solvent system and controlled by an active esterification modifier. Figure 3 These are digital photographs of lignocellulose raw materials, lignocellulose swollen with low co-solvent, and controllably modified nano-lignocellulose. Figure 4 The images are transmission electron microscope (TEM) images of nano-lignocellulose that have been fully swollen in a eutectic solvent system and controlled by an active esterification modifier, along with their calculated aspect ratios. Figure 5 The carboxyl content of nano-lignocellulose was determined by sodium hydroxide titration using four typical active esterification modifiers. Figure 6 These are transmission electron micrographs of nano-lignocellulose controlled by four typical active esterification modifiers and their corresponding average particle size distributions.

[0032] Figure 7 This is a schematic diagram illustrating the mechanism of controllable modification of nano-lignocellulose by four typical active esterification modifiers of this invention.

[0033] Figure 8 This is the technical roadmap of the present invention. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings, technical process steps, specific implementation conditions and materials in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1 Take 2 g of oven-dried eucalyptus bleached pulp (lignin content 2.4 wt%) and add it to 100 g of a eutectic solvent system consisting of zinc acetate / choline chloride and 10 wt% water. o After swelling at 90°C for 120 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 4 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 50 g of maleic anhydride was added as a modifier to adjust the content of fully swollen lignocellulose raw material to 2 wt% of the eutectic solvent and modifier. o After C modification for 60 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then mechanically dispersed in a blender for 5 min to finally obtain controllable modified nano-lignocellulose containing double bonds and carboxyl functional groups. Its average diameter was 3.47 nm, aspect ratio was 1800, carboxyl content was 1.412 mmol / g, and yield was 92.6%.

[0036] Example 2 Take 2 g of oven-dried bleached bamboo fiber pulp (lignin content 9.2 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / betaine and 20 wt% water. o After swelling at 95°C for 180 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 20 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 10 g of succinic anhydride was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 10 wt% of the eutectic solvent and modifier. o After C modification for 120 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then dispersed mechanically using an ultrafine grinding mill for 20 min to finally obtain controllably modified nano-lignocellulose containing carboxyl functional groups. Its average diameter was 3.61 nm, aspect ratio was 2910, carboxyl content was 2.02 mmol / g, and yield was 90.8%.

[0037] Example 3 Take 20 g of oven-dried *Phyllostachys edulis* parenchyma cell powder (lignin content 20.98 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / choline bromide and 5 wt% water. o After swelling for 180 min at C, 100 g of norbornenic anhydride was added to ensure the fully swollen lignocellulose raw material comprised 10 wt% of the eutectic solvent and modifier. oAfter C modification for 120 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then dispersed using a high-pressure homogenizing mill for 20 min to finally obtain controllably modified nano-lignocellulose containing carboxyl groups and bridged methylene functional groups. Its average diameter was 4.52 nm, aspect ratio was 1130, carboxyl content was 1.08 mmol / g, and yield was 86.2%.

[0038] Example 4 Take 2 g of oven-dried poplar wood powder (lignin content 25.1 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / tetramethylammonium bromide and 15 wt% water. o After swelling at C for 180 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 4 wt%. 50 g of phthalic anhydride was added to ensure that the fully swollen lignocellulose raw material accounted for 2 wt% of the eutectic solvent and modifier. 90 o After C modification for 120 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then mechanically dispersed using a colloid mill for 40 min to finally obtain controllably modified nano-lignocellulose containing carboxyl and benzene ring functional groups. Its average diameter was 5.19 nm, aspect ratio was 710, carboxyl content was 1.07 mmol / g, and yield was 80.5%.

[0039] Example 5 Take 0.1 g of oven-dried poplar bleached pulp (lignin content 1.9 wt%) and add it to 100 g of a eutectic solvent system consisting of zinc chloride / choline chloride and 10 wt% water. o After swelling at 60°C for 120 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 0.2 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 50 g of acetic anhydride was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 0.1 wt% of the eutectic solvent and modifier. o After C modification for 60 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then ultrasonically dispersed for 10 min to finally obtain controllably modified nano-lignocellulose containing acetyl functional groups. Its average diameter was 11.3 nm, aspect ratio was 860, and yield was 92.1%.

[0040] Example 6 Take 2 g of oven-dried poplar bleached pulp (lignin content 2.8 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / tetramethylammonium bromide and 10 wt% water. o After swelling for 30 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 20 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 90 g of maleic anhydride was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 2 wt% of the eutectic solvent and modifier. o After C modification for 5 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then mechanically dispersed in a blender for 10 min to finally obtain controllable modified nano-lignocellulose containing double bonds and carboxyl functional groups. Its average diameter was 4.8 nm, aspect ratio was 1230, carboxyl content was 0.91 mmol / g, and yield was 90.8%.

[0041] Example 7 Take 5 g of oven-dried wheat straw pulp (lignin content 18.2 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / tetrabutylammonium bromide and 10 wt% water. o After swelling at 90°C for 120 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 4 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 50 g of dodecenylsuccinic anhydride was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 2 wt% of the eutectic solvent and modifier. o After C modification for 60 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then mechanically dispersed using a colloid mill for 40 min to finally obtain controllably modified nano-lignocellulose containing double bonds, alkyl groups, and carboxyl functional groups. Its average diameter was 19.2 nm, aspect ratio was 580, carboxyl content was 0.82 mmol / g, and yield was 81.5%.

[0042] Example 8 Take 2 g of oven-dried poplar bleached pulp (lignin content 1.9 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / choline chloride. o After swelling at 90°C for 120 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 4 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 100 g of lactic acid was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 2 wt% of the eutectic solvent and modifier. oAfter C modification for 120 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then mechanically dispersed under high pressure for 20 min to finally obtain controllable modified nano-lignocellulose containing lactyl functional groups. Its average diameter was 13.2 nm, aspect ratio was 760, and yield was 88.4%.

[0043] Example 9 Take 5 g of oven-dried bleached bamboo fiber pulp (lignin content 8.5 wt%) and add it to 100 g of a eutectic solvent system composed of zinc chloride / choline chloride and 10 wt% water. o After swelling at 90°C for 120 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 10 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 50 g of oxalic acid was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 5 wt% of the eutectic solvent and modifier. o After C modification for 60 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then mechanically dispersed using a colloid mill for 40 min to finally obtain controllably modified nano-lignocellulose containing carboxyl functional groups. Its average diameter was 18.6 nm, aspect ratio was 510, carboxyl content was 1.35 mmol / g, and yield was 86.6%.

[0044] Example 10 Take 2 g of oven-dried poplar wood powder (lignin content 22.8 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / betaine and 10 wt% water. o After swelling at 90°C for 180 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 4 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 50 g of maleic acid was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 2 wt% of the eutectic solvent and modifier. o After C modification for 60 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then dispersed by ultrafine grinding for 20 min to finally obtain controllable modified nano-lignocellulose containing double bonds and carboxyl functional groups. Its average diameter was 17.5 nm, aspect ratio was 680, carboxyl content was 1.61 mmol / g, and yield was 84.7%.

[0045] Example 11 Take 2 g of oven-dried poplar bleached pulp (lignin content 1.7 wt%) and add it to 100 g of a eutectic solvent system composed of zinc acetate / tetramethylammonium bromide and 10 wt% water. o After swelling at 80°C for 120 min, excess eutectic solvent was squeezed out through a filter to adjust the content of fully swollen lignocellulose in the eutectic solvent to 4 wt%. The squeezed-out eutectic solvent was recycled. Subsequently, 50 g of citric acid was added as a modifier to ensure that the fully swollen lignocellulose raw material accounted for 2 wt% of the eutectic solvent and modifier. o After C modification for 120 min, excess eutectic solvent and modifier were squeezed out through a filter for recycling. A small amount of residual eutectic solvent and modifier were washed until neutral, and then ultrasonically dispersed for 20 min to finally obtain controllably modified nano-lignocellulose containing carboxyl functional groups. Its average diameter was 8.3 nm, aspect ratio was 1720, carboxyl content was 1.97 mmol / g, and yield was 90.8%.

[0046] Comparative Example 1 Take 2 g of oven-dried poplar bleached pulp (lignin content 2.4 wt%) and add it to a eutectic solvent system consisting of zinc acetate / choline chloride and 10 wt% water. o After being swelled at high temperature for 120 min, excess eutectic solvent was squeezed out through a filter for reuse, yielding fully swelled lignocellulose raw material. After washing the fully swelled lignocellulose raw material to neutrality with excess eutectic solvent, it was mechanically dispersed for 5 min using a high-speed blender. However, a large number of macroscopically sized lignocellulose fibers remained, and no surface-modified functional groups were found, making it impossible to prepare a controllable modified nano-lignocellulose dispersion.

[0047] Comparative Example 2 Take 2 g of oven-dried bamboo fiber bleached pulp (lignin content 9.2 wt%) and, without swelling in a eutectic solvent system, directly add succinic anhydride as a modifier to achieve a lignocellulose raw material content of 10 wt%. o C modification for 120 min. Due to the lack of a eutectic solvent, succinic anhydride cannot lower its melting point, thus preventing it from forming a stable solution and causing it to remain in solid powder form. Therefore, it is impossible to perform controlled esterification modification on lignocellulose raw materials.

[0048] Comparative Example 3 Take 2 g of oven-dried bleached wood pulp (lignin content 2.1 wt%) and add it directly to an esterification modification system composed of citric acid / choline chloride and 5 wt% water without swelling in a eutectic solvent system. oThe reaction was carried out at C for 120 min, and the excess esterification modification system was squeezed out through a filter for recycling. After washing away a small amount of residual eutectic solvent and modifier until neutral, ultrasonic mechanical dispersion was performed for 20 min. The prepared modified nano-lignocellulose still contained a large number of macroscopically sized lignocellulose fibers with an average diameter of 142 nm, an aspect ratio of 120, and a carboxyl content of 0.38 mmol / g, making it impossible to prepare a controllable modified nano-lignocellulose dispersion.

[0049] The embodiments involved in this invention are not limited to the above embodiments. Table 1 provides a brief list of the controllable modification of nano-lignocellulose in the eutectic solvent system involved in this invention. Referring to the experimental steps of Example 1, the effects of the type of lignocellulose raw material, the type of eutectic solvent, the water content, the swelling temperature and time, the type of esterification modifier, the modification temperature and time, the type of modified functional groups, and the mechanical dispersion method on the controllable modification of nano-lignocellulose were investigated. The main indicators such as the average diameter, aspect ratio, yield, and functional groups of the controllable modified nano-lignocellulose in the eutectic solvent system were statistically analyzed.

[0050] Table 1

[0051] ; Scanning electron microscope images of the lignocellulose raw material before and after full swelling in the eutectic solvent in Example 1 are shown below. Figure 1 As shown, the microstructure of unswelled lignocellulose raw material is dense and fibrous. However, after being fully swollen by a eutectic solvent, the surface of lignocellulose exhibits obvious fibrillation and erosion. This indicates that after swelling by the eutectic solvent, the dense natural structure of lignocellulose is destroyed and becomes loose, which can promote the efficiency of subsequent controllable esterification modification.

[0052] The projected electron image of the controllable modified nano-lignocellulose prepared in Example 1 after sufficient swelling in a eutectic solvent and efficient esterification with an esterification modifier is shown below. Figure 2 As shown, after sufficient swelling and functionalization modification with eutectic solvent and modifier, the stubborn natural structure and hydrogen bond network of lignocellulose are effectively destroyed, and nano-lignocellulose with an average diameter of only 3.47 nm can be prepared.

[0053] Macroscopic digital photographs of the lignocellulose raw material, the lignocellulose raw material fully swollen by the eutectic solvent, and the controllably modified nano-lignocellulose in Example 1 are shown below. Figure 3As shown, after sufficient swelling in the eutectic solvent, the macroscopic morphology of the lignocellulose raw material is colloidal, indicating that the eutectic solvent has fully penetrated into the interior of the lignocellulose raw material, achieving sufficient swelling in the eutectic solvent system. After controllable modification with a highly efficient esterification modifier, the macroscopic morphology of the nano-lignocellulose is a translucent gel, indicating that the lignocellulose raw material has undergone sufficient nanofiberization and the negative charge repulsion brought about by carboxylation produces an excellent nano-effect, thus exhibiting a translucent gel with high viscosity.

[0054] Scanning transmission microscopy of nano-lignocellulose prepared by fully swelling and controllably modifying the eutectic solvent in Example 1, and its corresponding major axis, for example... Figure 4 As shown, the aspect ratio as high as ~1800 indicates that the low eutectic solvent fully swells, which is beneficial for the esterification and functionalization of nano-lignocellulose to achieve complete nanofiberization, thus resulting in an excellent aspect ratio.

[0055] The carboxyl content of the nano-lignocellulose prepared by fully swelling and controllably modifying the eutectic solvent in Examples 1-4, as determined by conductivity titration, is as follows: Figure 5 As shown, all four types of carboxylated nanocellulose exhibited high carboxyl content, with the carboxylated nanocellulose prepared by succinic anhydride modification in Example 2 reaching a carboxyl content as high as 2.02 mmol / g. This indicates that the lignocellulose, after being fully swollen in a eutectic solvent system, significantly improved its accessibility and the activity of hydroxyl groups on the cellulose surface, thereby enhancing the esterification modification efficiency and resulting in a higher carboxyl content.

[0056] Transmission electron microscopy (TEM) images and diameter distributions of nano-lignocellulose prepared by fully swelling and controllably modifying the eutectic solvent in Examples 1-4 are shown below. Figure 6 As shown, the excellent microstructure of nano-lignocellulose can be achieved by flexibly changing the esterification modifier. This is because the sufficient swelling of the eutectic solvent destroys its stubborn natural structure while maintaining the crystal structure and structural strength. It also breaks the stubborn hydrogen bond network while retaining the surface active hydroxyl groups of lignocellulose and improving its accessibility. This plays a key role in the subsequent controllable functionalization modification and mechanical dispersion process.

[0057] The mechanism diagrams of the four classic carboxylated nanocellulose modifications in Examples 1-4 are shown below. Figure 7 As shown, this indicates that lignocellulose raw materials, after being swollen at high temperatures, can undergo ring-opening esterification modification with various acid anhydride modifiers, introducing active functional groups such as carboxyl groups, double bonds, benzene rings, and bridged methylene groups.

[0058] Example 1 and Comparative Example 1 serve as control groups. Under the same conditions of lignocellulose raw material type, eutectic solvent composition, and swelling temperature and time, lignocellulose raw materials that have not undergone controlled modification by fully swelling in the eutectic solvent with an esterifying agent lack surface-modified functional groups and cannot be prepared into controllably modified nano-lignocellulose dispersions. This indicates that while the eutectic solvent's full swelling of the lignocellulose raw material disrupts the stubborn natural structure, it maintains the crystal structure and structural strength. It breaks the stubborn hydrogen bond network while retaining the surface-active hydroxyl groups of lignocellulose and improving their accessibility. This is the key basis for improving the efficiency and reactivity of subsequent controllable functionalization modification. This is a key difference that clearly distinguishes this patent from existing patent technologies.

[0059] Example 2 and Comparative Example 2 serve as control examples. Under the same conditions of lignocellulose raw material type, esterification modifier type and ratio, and modification temperature and time, lignocellulose raw material that has not been sufficiently swelled in a eutectic solvent cannot be formulated into a stable and dispersed solvent system with a high-melting-point modifier. Therefore, it is impossible to controllably modify the lignocellulose raw material to prepare nano-lignocellulose. This indicates that a eutectic solvent can lower the melting point of a single component, reducing the melting point of a high-melting-point active modifier and allowing it to exhibit excellent compatibility with the eutectic solvent. This enables the efficient functional modification of lignocellulose by various active esterification modifiers with high melting points and poor solubility, thus satisfying the need for flexible and controllable preparation of modified nano-lignocellulose under mild conditions. This is a key difference that clearly distinguishes this patent from existing patent technologies.

[0060] Example 11 and Comparative Example 3 serve as control examples. Under the same conditions of lignocellulose raw material type, esterification modifier type and ratio, lignocellulose raw material that has not undergone sufficient swelling treatment with a eutectic solvent cannot be used to prepare nano-lignocellulose with the diameter and aspect ratio required by this patent claim under the mild conditions of reaction time and temperature. This indicates that sufficient swelling of lignocellulose raw material by a eutectic solvent helps to improve the accessibility and reactivity of the active hydroxyl groups in lignocellulose, thereby improving the modification efficiency of controllable esterification modification and achieving the preparation of controllable modified nano-lignocellulose under mild conditions. This is a key difference that clearly distinguishes this patent from existing patent technologies.

Claims

1. A method for preparing controllably modified nano-lignocellulose, characterized in that, Includes the following steps: S1. The lignocellulose raw material is swollen using a eutectic solvent, wherein the lignocellulose raw material content in the eutectic solvent is 0.1-20 wt%, and excess eutectic solvent is separated for reuse; the eutectic solvent consists of a hydrogen bond donor, a hydrogen bond acceptor, and water; the hydrogen bond donor is zinc acetate or zinc chloride, the hydrogen bond acceptor is any one or a combination of two or more of choline chloride, choline bromide, tetramethylammonium bromide, tetrabutylammonium bromide, or betaine, and the water content is 0-20 wt%. S2. Add an esterification modifier to the fully swollen lignocellulose / eutectic solvent mixture obtained in step S1 and adjust the lignocellulose concentration to 1-20 wt% for mild and controllable esterification modification. Separate excess esterification modifier for reuse, and then prepare a surface-modified nano-lignocellulose dispersion through mechanical processing. The modifier is an acid anhydride and / or an organic acid. The mechanical processing is one or more of the following mechanical dispersion treatments: cell wall breaking machine, colloid mill, ultrasonication, ultrafine grinding, and high-pressure homogenization.

2. The method for preparing controllably modified nano-lignocellulose according to claim 1, characterized in that: The lignocellulose raw materials mentioned in step S1 are cellulose, wood, grass, bamboo and hemp.

3. The method for preparing controllably modified nano-lignocellulose according to claim 1, characterized in that: In step S2, the acid anhydrides are any one or a combination of two or more of maleic anhydride, succinic anhydride, phthalic anhydride, norbornene anhydride, dodecenylsuccinic anhydride, and acetic anhydride; the organic acids are any one or a combination of two or more of lactic acid, oxalic acid, maleic acid, and citric acid.

4. The method for preparing controllably modified nano-lignocellulose according to claim 1, characterized in that: The swelling reaction temperature in step S1 is 60-95°C. o C, reaction time is 30-180 min.

5. The method for preparing controllably modified nano-lignocellulose according to claim 1, characterized in that: The esterification modification reaction temperature in step S2 is 60-95°C. o C, reaction time is 5-120 min.

6. The method for preparing controllably modified nano-lignocellulose according to claim 4, characterized in that: The surface of the anhydride-modified nano-lignocellulose contains active carboxyl groups and functional groups in the same molar ratio, and the content of the active carboxyl groups and functional groups is 0.8-2.1 mmol / g; the functional groups are any one or a combination of two or more of double bonds, benzene rings, alkyl groups, acetyl groups, and bridged methylene groups.

7. The method for preparing controllably modified nano-lignocellulose according to claim 4, characterized in that: The surface of the nano-lignocellulose modified with organic acid contains active carboxyl groups, and the content of the active carboxyl groups is 1.0-2.0 mmol / g.

8. A controllable modified nano-lignocellulose, characterized in that: It was prepared using the method for preparing controllable modified nano-lignocellulose as described in any one of claims 1 to 8.

9. The controllable modified nano-lignocellulose according to claim 9, characterized in that: The diameter of the nano-lignocellulose is 3-20 nm, the aspect ratio is 500-3000, the yield is 80-93%, and the lignin content is 0-25%.

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