阅读说明：本技术 采用熔盐体系制备纳米级再生纤维素的方法 (Method for preparing nano-grade regenerated cellulose by adopting molten salt system ) 是由 李滨 黄仲雷 刘超 吴美燕 崔球 于 2020-08-14 设计创作，主要内容包括：本发明公开了一种采用熔盐体系制备纳米级再生纤维素的方法,具体包括以下步骤：(1)将纤维素原料加入到熔盐体系中加热溶解,得到均匀透明的纤维素-熔盐溶液；(2)向纤维素-熔盐溶液中添加适量再生溶剂,固液分离,得到纳米级再生纤维素；(3)将分离得到的再生纤维素固体用水清洗,分离得到的液体蒸发浓缩回收再生溶剂,并得到再生的熔盐溶液；(4)将清洗后的再生纤维素配置成纤维素分散液,高压均质处理,即可得到稳定分散的纳米级再生纤维素。本发明制得的纳米级再生纤维素得率为96％-99％；而且成本低廉、过程绿色清洁可持续,还能得到不同聚集态的纳米纤维素材料,对于纳米纤维素工业进程的发展具有重要的推动作用。(The invention discloses a method for preparing nano regenerated cellulose by adopting a molten salt system, which specifically comprises the following steps: (1) adding a cellulose raw material into a molten salt system, heating and dissolving to obtain a uniform and transparent cellulose-molten salt solution; (2) adding a proper amount of regenerated solvent into the cellulose-molten salt solution, and carrying out solid-liquid separation to obtain nano regenerated cellulose; (3) washing the regenerated cellulose solid obtained by separation with water, evaporating and concentrating the liquid obtained by separation to recover a regenerated solvent, and obtaining a regenerated molten salt solution; (4) and preparing the cleaned regenerated cellulose into cellulose dispersion liquid, and carrying out high-pressure homogenization treatment to obtain the stably dispersed nano-grade regenerated cellulose. The yield of the nano-grade regenerated cellulose prepared by the method is 96-99 percent; the method has the advantages of low cost, green, clean and sustainable process, capability of obtaining nano-cellulose materials in different aggregation states, and important promotion effect on the development of the industrial process of nano-cellulose.)

1. The method for preparing the nano regenerated cellulose by adopting a molten salt system is characterized by comprising the following steps: the method comprises the following steps:

(1) adding a proper amount of cellulose raw materials into a molten salt system, heating and stirring until the cellulose raw materials are dissolved to obtain a uniform and transparent cellulose-molten salt solution; the molten salt system is a liquid molten salt hydrate, and the weight fraction of inorganic salt in the molten salt system is 20-90%;

(2) adding a proper amount of regeneration solvent into the cellulose-molten salt solution obtained in the step (1), and carrying out solid-liquid separation to obtain nano-grade regenerated cellulose; the regeneration solvent is water, ethanol or N, N-dimethylacetamide; the weight ratio of the regenerated solvent to the cellulose-molten salt solution is 1-20: 1.

2. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 1, wherein: the solid-liquid mass ratio of the cellulose raw material to the molten salt system in the step (1) is 1:1-100, the heating and stirring temperature is 50-180 ℃, the heating and stirring speed is 300-1000rpm, and the heating and stirring time is 0.1-72 h; the weight ratio of the regeneration solvent to the cellulose-molten salt solution in the step (2) is 1-15: 1.

3. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 2, characterized in that: when the regeneration solvent in the step (2) is water, obtaining cellulose nanocrystalline or cellulose nanospheres with a cellulose II type structure; when the regeneration solvent is absolute ethyl alcohol, cellulose nano-fibrils with cellulose II-type structures are obtained; when the regeneration solvent is N, N-dimethylacetamide, the cellulose nano-fibril with an amorphous structure is obtained.

4. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 2, characterized in that: in the molten salt system in the step (1), the molten salt is Li⁺、Na⁺、Mg²⁺、K⁺、Ca²⁺、Zn²⁺、Rb⁺、Ag⁺、Cs⁺And NH₄ ⁺And the complex salt hydrate is one or more of hydrates of chloride, bromide, iodate, perchlorate, nitrate and thiocyanate.

5. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 2, characterized in that: in the molten salt system in the step (1), the molten salt is Li⁺、Mg²⁺、Ca²⁺、Zn²⁺And one or more of chloride, bromide and perchlorate hydrates.

6. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 2, characterized in that: in the molten salt system in the step (1), the molten salt is a hydrate of zinc chloride, calcium chloride, magnesium chloride, lithium bromide or lithium perchlorate.

7. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 2, characterized in that: the cellulose raw material is one of microcrystalline cellulose, softwood dissolving pulp, hardwood dissolving pulp, gramineae dissolving pulp, cotton pulp and commodity bleaching pulp.

8. The method for preparing nano-scale regenerated cellulose using a molten salt system according to any one of claims 1 to 7, characterized in that: the method further comprises the steps of:

(3) washing the separated nano regenerated cellulose solid with water, evaporating and concentrating the separated liquid, and recovering a regenerated solvent to obtain a regenerated molten salt hydrate;

(4) and (4) preparing the nano regenerated cellulose obtained in the step (3) into cellulose dispersion liquid, and carrying out high-pressure homogenization treatment to obtain the nano regenerated cellulose stably dispersed in a water phase.

9. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 8, wherein: the solid-liquid separation in the step (2) is centrifugal separation or filtration; the regenerated molten salt hydrate obtained in the step (3) can be recycled and used for preparing the cellulose-molten salt solution in the step (1).

10. The method for preparing nano-scale regenerated cellulose by using a molten salt system according to claim 8, wherein: the regenerated cellulose dispersion liquid in the step (4) has the mass concentration of 0.1-10%, and the high-pressure homogenization treatment conditions are as follows: homogenizing under 30-150MPa for 1-20 times.

Technical Field

The invention belongs to the field of materials, relates to the field of preparation of natural high polymer materials, and particularly relates to a method for preparing nano regenerated cellulose by adopting a molten salt system.

Background

In recent years, resource and environmental issues have been receiving more and more attention, and there is a trend of necessity to develop and utilize renewable resources instead of conventional fossil resources. Cellulose is the most abundant biopolymer on earth, mainly present in the cell walls of higher plants; therefore, the method has the advantages of wide raw material source, no toxicity, reproducibility, degradability and the like. The nano-cellulose as a nano-scale material has the advantages of high specific surface area, high length-diameter ratio and low density, and also has high Young modulus, high tensile strength, high crystallinity, low thermal expansion coefficient and the like. Based on the characteristics, the nano-cellulose has wide application prospect in the fields of reinforcing materials, catalyst carriers, drug carriers, composite materials, packaging materials, development of photoelectric devices and the like.

Nanocellulose prepared from plant cellulose fibers is mainly classified into two types according to the morphology: cellulose Nanofibrils (CNFs) and Cellulose Nanocrystals (CNCs). Among them, the simplest method for preparing Cellulose Nanofibrils (CNFs) is a mechanical method. In 1983 J.appl.Polymer.Sci. reported that Herrick and Casebier et al prepared cellulose nanofibrils based on wood pulp for the first time using high pressure homogenization. Research on the preparation of cellulose nanofibrils with high aspect ratios by using high-intensity ultrasonic cavitation has also been reported. Chen et al reported in carbon hydrate Polymers 2011 that cellulose nanofibrils with type I structures were prepared from bamboo fibers by a method combining chemical pretreatment and high intensity ultrasound. However, the above methods require intensive mechanical treatment and are very energy-consuming and substantially impossible to use directly in industrial production.

The method commonly used for the preparation of Cellulose Nanocrystals (CNCs) is the strong acid hydrolysis method (sulfuric acid, hydrochloric acid, etc.), which is mainly obtained by hydrolyzing amorphous and partially crystalline regions in cellulose. The strong acid adopted by the method has high concentration, so that the method is very easy to cause equipment corrosion, and can generate a large amount of waste acid, thereby causing various environmental problems. In order to solve the problem, researchers propose a technical scheme for preparing the cellulose nanocrystal by combining ionic liquid swelling fibers with a micro-acid system. Lazko et al (Cellulose,2014,2,4195-4207) use ionic liquid [ Bmim ] Cl as a solvent to swell the raw material before sulfuric acid hydrolysis, increasing the hydrolysis activity of Cellulose. Compared with the traditional sulfuric acid method, the reaction conditions of the cellulose treated by the ionic liquid and sulfuric acid are milder, and the amount of the sulfuric acid used in the reaction process is less. The invention patent application 201910544934.4 discloses a method for preparing cellulose nanocrystals by an inorganic molten salt-micro acid co-promotion system, which is to dissociate the amorphous region of cellulose by the inorganic molten salt-micro acid co-promotion system to obtain the cellulose nanocrystals. The method solves the problem that strong acid corrodes equipment in the preparation process of the cellulose nanocrystals, realizes the repeated recycling of the system, and reduces the difficulty of acid sewage treatment; it is undeniable that the above reaction is still hydrolyzed in an acid solution, only the concentration of the acid is reduced, and the cellulose raw material is degraded into cellulose nanocrystals by the acid before dissolution.

In general, current research on the preparation of nanocellulose focuses on cellulose type I, and there are mainly the following problems: difficult recovery of chemical reagents, expensive chemicals, long reaction time, environmental pollution and high energy consumption. In addition, as the industrial process of nanocellulose is continuously advanced, the demand for the preparation process of nanocellulose with different aggregation structures is continuously increased. Based on this, research for preparing type II nanocellulose is also being increasingly made. Kim et al (Polymer,2006,47(14),5097-5107) dissolve cellulose in LiCl/DMAc and NMMO solvent systems to prepare cellulose nanowires with high aspect ratio by an electrospinning method. However, such organic solvents are expensive, the recovery cost is high, and the prepared nano-cellulose has a large particle size. Phantong et al (Cellulose,2017,24(5),2083-2093) prepared Cellulose nanofibrils with diameters of 10-15nm by using an ionic liquid combined ball milling process. The method mainly comprises the steps of preparing nano cellulose by destroying hydrogen bonds among cellulose molecules; although the ionic liquid has high dissolving capacity, the cost is high, and the requirement on the moisture content in the using process is strict, so that the industrial popularization of the ionic liquid is restricted.

Therefore, the method for preparing the nano-grade cellulose with green, clean, efficient and economical development process still is an urgent problem to be solved in the process of nano-grade cellulose industry, and has important significance for the development of the industry.

Disclosure of Invention

In view of the above problems of the prior art, it is an object of the present invention to provide a novel method for preparing nano-sized regenerated cellulose using a molten salt system. The method is simple to operate, low in cost and environment-friendly, all the solvents are easy to recycle, and the most key technical problem is solved for the industrial process.

The technical scheme of the invention is as follows:

a method for preparing nano-grade regenerated cellulose by adopting a molten salt system comprises the following steps:

(1) adding a proper amount of cellulose raw materials into a molten salt system, heating and stirring until the cellulose raw materials are dissolved to obtain a uniform and transparent cellulose-molten salt solution; the molten salt system is a liquid molten salt hydrate, the amount of substances combined with water in 1mol of the molten salt system is 3-6mol, and the weight fraction of inorganic salt in the molten salt system is 20-90 percent by weight. The solid-liquid mass ratio of the cellulose raw material to the molten salt system is 1:1-100, the heating and stirring temperature is 50-180 ℃, the heating and stirring speed is 300-1000rpm, and the heating and stirring time is 0.1-72 h. The molten salt is Li⁺、Na⁺、Mg²⁺、K⁺、Ca²⁺、Zn²⁺、Rb⁺、Ag⁺、Cs⁺And NH₄ ⁺And the complex salt hydrate is one or more of hydrates of chloride, bromide, iodate, perchlorate, nitrate and thiocyanate. It is critical that the molten salt exists in a hydrate state in the molten salt system. This is because the salt cation is generally more likely to bind to water and only binds to the hydroxyl groups of cellulose when saturated to coordinate with water.

(2) And (2) adding a proper amount of regeneration solvent into the cellulose-molten salt solution obtained in the step (1), and performing solid-liquid separation to obtain the nano-grade regenerated cellulose. The regeneration solvent is water, ethanol or N, N-dimethylacetamide (DMAc); the weight ratio of the regenerated solvent to the cellulose-molten salt solution is 1-20: 1. The solid-liquid separation method is centrifugal separation or filtration. Researchers find that the regenerated cellulose in a molten salt system and the regenerated solvent have important influence on the crystal structure and the appearance of the regenerated cellulose for the first time. When the regeneration solvent is water, cellulose nanocrystalline or cellulose nanospheres with a cellulose II type structure are obtained; when the regeneration solvent is absolute ethyl alcohol, cellulose nano-fibrils with cellulose II-type structures are obtained; when the regeneration solvent is N, N-dimethylacetamide (DMAc), cellulose nanofibrils of amorphous structure are obtained. It is an unexpected finding in research that cellulose is regenerated to obtain nano-scale cellulose after being dissolved in a molten salt system. More surprisingly, different regenerated solvents can obtain nanometer regenerated cellulose with different shapes and crystal structures.

Wherein the cellulose raw material is one of microcrystalline cellulose, softwood dissolving pulp, hardwood dissolving pulp, gramineae dissolving pulp, cotton pulp and commodity bleaching pulp.

Preferably, the weight ratio of the regeneration solvent to the cellulose-molten salt solution is 1-15: 1.

Preferably, the molten salt is Li⁺、Mg²⁺、Ca²⁺、Zn²⁺And one or more of chloride, bromide and perchlorate hydrates.

More preferably, the molten salt is a hydrate of zinc chloride, calcium chloride, magnesium chloride, lithium bromide or lithium perchlorate.

The method for preparing the nano regenerated cellulose by adopting the molten salt system can also comprise the following steps:

(3) and washing the separated nano regenerated cellulose solid with water, evaporating and concentrating the separated liquid, recovering the regenerated solvent, and simultaneously obtaining the regenerated molten salt hydrate. And (3) recycling the regenerated solvent and the molten salt hydrate obtained by recovery, wherein the molten salt hydrate can be used for preparing the cellulose-molten salt solution in the step (1). And the regenerated solvent can be used for preparing the nano-grade regenerated cellulose in the step (2). The recycling not only reduces the cost, but also is environment-friendly, and accords with the large trend of the industrial development of clean production.

(4) And (4) preparing the nano regenerated cellulose obtained in the step (3) into cellulose dispersion liquid, and carrying out high-pressure homogenization treatment to obtain the nano regenerated cellulose capable of being stably dispersed in a water phase. The mass concentration of the regenerated cellulose dispersion liquid is 0.1-10%, and the conditions of the high-pressure homogenization treatment are as follows: homogenizing under 30-150MPa for 1-20 times.

The preparation principle is as follows:

(1) and (3) dissolving: the metal salt cation in the molten salt and the oxygen atom in the hydroxyl on the surface of the cellulose generate electrostatic interaction, so that the cellulose hydroxyl and the metal salt cation are coordinated to replace the water molecule coordinated with the metal cation in the molten salt. The coordination of the hydroxyl and the metal salt breaks the hydrogen bond action between cellulose molecular chains, so that the cellulose molecular chains are dissociated. Meanwhile, a part of metal salt cations permeate into the cellulose crystallization area part, and the dissociation of cellulose molecular chains is further promoted. In addition, the anions in the molten salt are combined with hydrogen atoms in the cellulose hydroxyl groups, so that the reformation of hydrogen bonds among cellulose molecular chains is avoided. By combining all the factors, the cellulose is dissolved into the molten salt solution to form a uniform and transparent cellulose-molten salt solution.

(2) And (3) a regeneration process: the regenerated solvent is added into the cellulose-molten salt solution, a large amount of regenerated solvent quickly dilutes the molten salt solution and permeates into a coordination system formed by cellulose and molten salt cations, and the coordination combination of cellulose hydroxyl and the molten salt cations is broken, so that the cellulose is released. The released cellulose molecular chains quickly generate hydrogen bond recombination among the molecular chains, and the generated cellulose molecular chains are re-aggregated to form the nano regenerated cellulose with different crystal structure. Unpredictably, the nano-scale cellulose is obtained by dissolving and regenerating molten salt, and the type of the regenerated solvent determines the crystal structure and the morphology of the regenerated cellulose.

The invention has the beneficial effects that:

(1) the inorganic molten salt system adopted by the invention has low cost and easy preparation, can be recycled, and the whole process is green, clean and sustainable, thereby having important promotion effect on the development of the industrial process of the nano cellulose.

(2) The preparation method provided by the invention has the advantages of high product yield (more than 95%), mild conditions, simplicity in operation, no need of adding any catalyst/acid-base reagent, great reduction in production cost, and solving of the problems of unclean process and low product yield existing in the preparation of nano-cellulose in the prior art.

(3) The invention utilizes a molten salt system as a cellulose solvent to prepare the nano regenerated cellulose, obtains nano cellulose material products with different aggregation states, and pushes the cellulose nano material to be marketized.

Drawings

FIG. 1 is a process flow diagram employed in the present invention.

FIG. 2 is an XRD pattern of regenerated cellulose and microcrystalline cellulose of nanometer scale (A) prepared in example 1(C), example 4(B) and example 7(D) of the present invention. Wherein B is the cellulose nanofibrils with a cellulose II type structure obtained in example 4 with ethanol as the regeneration solvent; c is cellulose nanocrystal with cellulose II structure obtained by using water as regeneration solvent in example 1; d is the amorphous cellulose nanofibrils obtained in example 7 with DMAc as regeneration solvent.

FIG. 3 is a TEM image of cellulose nanocrystals prepared in example 2 of the present invention.

Fig. 4 is a TEM image of the cellulose nanosphere prepared in example 3 of the present invention.

FIG. 5 is a TEM image of cellulose nanofibrils prepared according to example 4 of the invention.

FIG. 6 is a TEM image of cellulose nanofibrils prepared according to example 5 of the invention.

FIG. 7 is a TEM image of cellulose nanocrystals prepared in example 6 of the present invention.

FIG. 8 is a TEM image of cellulose nanofibrils prepared according to example 7 of the present invention.

Detailed Description

The present invention will be further described with reference to the following examples.