CN111793223A - Method for preparing nano-grade regenerated cellulose by adopting molten salt system - Google Patents
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Abstract
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.
Description
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.
Example 1:
100g of a molten salt system containing 54% (w/w) LiCl was prepared exactly with pure water. Accurately weighing 1g of microcrystalline cellulose powder, wherein the solid-to-liquid ratio is 1: 100, adding the mixture into a LiCl molten salt system; the temperature is raised to 95 ℃, and the cellulose is completely dissolved after stirring for 8 hours at the rotating speed of 600rpm, so as to obtain uniform cellulose-molten salt solution. Then, the cellulose-molten salt solution was poured into 200g of water, and filtered to obtain regenerated cellulose. Then re-dispersing the regenerated cellulose obtained by separation in 250g of pure water to prepare regenerated cellulose dispersion liquid; homogenizing for 20 times at 60MPa by a high-pressure homogenizer to obtain colloidal cellulose nanocrystal dispersion system.
The yield of the cellulose nanocrystal obtained in the embodiment is 98.2% (relative to the original oven-dried microcrystalline cellulose), the crystal structure of the prepared cellulose nanocrystal is obviously different from that of the cellulose raw material, and the crystallinity of the cellulose nanocrystal is reduced from 77.34% of the microcrystalline cellulose to 58.08%. The microcrystalline cellulose as the raw material has characteristic diffraction peaks of cellulose type I at 2 θ of 14.9 °, 16.5 °, 22.5 ° and 34.5 °. The regenerated cellulose nanocrystal only has stronger peaks at 2 theta of 12.3 degrees, 20.1 degrees and 21.9 degrees, which are characteristic diffraction peaks of cellulose II; this indicates that the regenerated fibrillated cellulose has undergone a transformation in its crystalline form, which is also responsible for its greatly reduced crystallinity. See in particular the XRD pattern of fig. 2.
Example 2:
accurately preparing 100g of ZnCl containing 25% (w/w) by using pure water2The molten salt system of (1). Weighing absolutely dry 2g of bleached softwood pulp accurately, wherein the solid-to-liquid ratio is 1: 50, adding the mixture into a ZnCl molten salt system; the temperature is raised to 105 ℃, and the cellulose is completely dissolved after stirring for 2 hours at the rotating speed of 300rpm, so as to obtain uniform cellulose-molten salt solution. Then pouring the cellulose-molten salt solution into 1000g of pure water, and carrying out centrifugal treatment to obtain regenerated cellulose. Then re-dispersing the regenerated cellulose obtained by separation in 300g of pure water to prepare regenerated cellulose dispersion liquid; homogenizing for 15 times at 60MPa by high pressure homogenizer to obtainCellulose nanocrystals.
The yield of cellulose nanocrystals prepared in this example was 97.4% (relative to the original oven dried bleached softwood pulp). The cellulose nanocrystalline with uniform grain diameter is prepared, and the diameter of the prepared cellulose nanocrystalline is 5-20nm and the length of the prepared cellulose nanocrystalline is 50-120nm according to TEM pictures and statistics by combining software. See in particular the transmission electron micrograph in figure 3.
Example 3:
accurately preparing 100g of LiClO containing 85% (w/w) by using pure water4The molten salt system of (1). Accurately weighing oven-dried 12.5g bleached hardwood pulp with a solid-to-liquid ratio of 1: 8 to LiClO4In a molten salt system, the temperature is raised to 115 ℃, and the cellulose is completely dissolved after continuously stirring for 0.5 hour at the rotating speed of 400rpm, so as to obtain uniform cellulose-molten salt solution. The cellulose-molten salt solution was then poured into 1500g of pure water to immediately obtain a flocculent regenerated cellulose. Then filtering, and re-dispersing the regenerated cellulose obtained by separation in 400g of absolute ethyl alcohol to prepare regenerated cellulose suspension; and homogenizing for 15 times under 100MPa by a high-pressure homogenizer to obtain the cellulose nanospheres.
The cellulose nanosphere yield produced in this example was 99.0% (relative to the original oven dried bleached hardwood pulp). The cellulose nanospheres with uniform particle sizes are prepared, and the diameter of the prepared cellulose nanospheres is 50-160nm according to TEM pictures and statistics by combining software. See in particular the transmission electron micrograph in figure 4.
Example 4:
100g of a molten salt system containing 64% (w/w) LiBr was prepared accurately with pure water. Accurately weighing absolutely dry 1g of dissolving pulp, wherein the solid-liquid ratio is 1: 100, adding the mixture into a LiBr molten salt system, heating to 125 ℃, and continuously stirring for 0.5 hour at the rotating speed of 1000rpm to completely dissolve cellulose to obtain a uniform cellulose-molten salt solution. Then pouring the cellulose-molten salt solution into 600g of absolute ethyl alcohol, and carrying out centrifugal separation to obtain regenerated cellulose; re-dispersing the regenerated cellulose obtained by separation in 300g of pure water to prepare regenerated cellulose suspension; and homogenizing the desalted cellulose suspension for 13 times at 60MPa by using a high-pressure homogenizer to obtain a colloidal cellulose nanofibril dispersion system.
The cellulose nanofibril yield produced in this example was 97.1% (relative to the original oven dried dissolving pulp). Cellulose nanofibrils with uniform particle sizes are prepared, and the diameter of the prepared cellulose nanofibrils is 5-20nm and the length of the prepared cellulose nanofibrils is about 1.5 mu m according to TEM pictures and statistics of software. See in particular the transmission electron micrograph in figure 5. The crystal structure of the prepared cellulose nanocrystal is obviously different from that of a cellulose raw material, and the crystallinity of the cellulose nanocrystal is reduced from 77.34% to 58.02% of microcrystalline cellulose. The microcrystalline cellulose as the raw material has characteristic diffraction peaks of cellulose type I at 2 θ of 14.9 °, 16.5 °, 22.5 ° and 34.5 °. The cellulose nanocrystals obtained by regeneration showed strong peaks only at 2 θ ═ 12.3 °, 20.1 ° and 21.9 °, which are characteristic diffraction peaks of cellulose type II, indicating that the crystalline form of fibrillated cellulose has been transformed. See in particular the XRD pattern of fig. 2.
Example 5:
accurately preparing 100g of CaCl containing 68% (w/w) by using pure water2The molten salt system of (1). Accurately weighing oven-dried 2g of broadleaf wood dissolving pulp, wherein the solid-liquid ratio is 1: 50, adding into CaCl2In a molten salt system, the temperature is raised to 125 ℃, and the cellulose is completely dissolved after continuously stirring for 2 hours at the rotating speed of 500rpm, so as to obtain uniform cellulose-molten salt solution. Then pouring the cellulose-molten salt solution into 300g of absolute ethyl alcohol, and filtering to obtain regenerated cellulose. Then re-dispersing the regenerated cellulose obtained by separation in 300g of absolute ethyl alcohol to prepare regenerated cellulose dispersion liquid; homogenizing for 2 times at 150MPa by a high-pressure homogenizer to obtain a colloidal cellulose nanofibril dispersion system.
The cellulose nanofibril yield produced in this example was 97.5% (relative to the original oven dried dissolving pulp). Cellulose nanofibrils with uniform particle sizes are prepared, and the diameter of the prepared cellulose nanofibrils is 5-20nm and the length of the prepared cellulose nanofibrils is about 1.5 mu m according to TEM pictures and statistics of software. See in particular the transmission electron micrograph in figure 6.
Example 6 (molten salt recovery and reuse:)
Evaporating and concentrating the filtered liquid in the embodiment 1 to obtain LiCl inorganic molten salt; this was then used to formulate 100g of a molten salt system containing 60% (w/w) LiCl. Accurately weighing 1g of microcrystalline cellulose powder, wherein the solid-to-liquid ratio is 1: 100, adding the mixture into a LiCl molten salt system, heating to 95 ℃, and continuously stirring at the rotating speed of 600rpm for 8 hours to completely dissolve cellulose to obtain a uniform cellulose-molten salt solution. Then, the cellulose-molten salt solution was poured into 100g of pure water, and filtered to obtain regenerated cellulose. And re-dispersing the regenerated cellulose obtained by separation in 250g of pure water to prepare regenerated cellulose suspension, and homogenizing for 20 times at 60MPa by using a high-pressure homogenizer to obtain a colloidal cellulose nanocrystal dispersion system.
The yield of cellulose nanocrystals produced in this example was 98.1% (relative to the original oven dried waste pulp). The cellulose nanocrystalline with uniform grain diameter is prepared, and the diameter of the prepared cellulose nanocrystalline is 5-20nm and the length of the prepared cellulose nanocrystalline is about 50-120nm according to TEM pictures and statistics by combining software. See in particular the transmission electron micrograph in figure 7. The effect of the regenerated molten salt recycled for preparing the nano regenerated cellulose is proved to be unchanged.
Example 7:
100g of a molten salt system containing 64% (w/w) LiBr was prepared accurately with pure water. Accurately weighing absolutely dry 1g of dissolving pulp, wherein the solid-liquid ratio is 1: 100, adding the mixture into a LiBr molten salt system, heating to 180 ℃, and continuously stirring at the rotating speed of 1000rpm for 0.1 hour to completely dissolve cellulose to obtain a uniform cellulose-molten salt solution. Then pouring the cellulose-molten salt solution into 300g DMAc, and centrifugally separating to obtain regenerated cellulose. Then re-dispersing the regenerated cellulose obtained by separation in 300g of pure water to prepare regenerated cellulose suspension; and homogenizing the desalted cellulose suspension for 15 times at 30MPa by a high-pressure homogenizer to obtain a colloidal cellulose nanofibril dispersion system.
The cellulose nanofibril yield produced in this example was 97.1% (relative to the original oven dried dissolving pulp). See in particular the transmission electron micrograph in figure 8. The crystal form structure of the prepared cellulose nano-fibril is obviously different from that of a cellulose raw material, and only one hump appears, which is a diffraction peak of an amorphous structure of cellulose, and indicates that the crystal form of regenerated cellulose is transformed. See in particular the XRD pattern of fig. 2.
Example 8:
100g of a molten salt system containing 25% (w/w) LiBr was prepared accurately with pure water. Accurately weighing absolute dry 1.25g of dissolving pulp, wherein the solid-liquid ratio is 1: 80, adding the mixture into a LiBr molten salt system, heating to 60 ℃, and continuously stirring at the rotating speed of 900rpm for 70 hours to completely dissolve cellulose to obtain a uniform cellulose-molten salt solution. Then pouring the cellulose-molten salt solution into 1000g of DMAc, and centrifugally separating to obtain regenerated cellulose. Then re-dispersing the regenerated cellulose obtained by separation in 15g of pure water to prepare regenerated cellulose suspension; and homogenizing the desalted cellulose suspension for 5 times at 30MPa by a high-pressure homogenizer to obtain a colloidal cellulose nanofibril dispersion system.
The cellulose nanofibrils produced in this example were produced in a yield of 99.0% (relative to the original oven dried dissolving pulp). The crystal structure of the obtained nano regenerated cellulose is amorphous.
In the above embodiment, XRD and TEM microscopic characterization is performed on the nanocellulose product, and the specific detection method is as follows:
(1) x-ray diffraction analysis: the freeze-dried cellulose raw material and the regenerated cellulose samples were analyzed by X-ray diffraction using an X-ray diffractometer (Bruker Discover D8, Bruker Co., Germany). The operating voltage is set to be 40kV, the filament current is 40mA, the scanning angle is 5-50 degrees, and the scanning speed is 4 degrees/min. Based on the reference textileres.j., 29: 786-794 (1959), the crystallinity (CrI) of the sample can be calculated by the formula CrI% ((I200-Iam)/I200 × 100%). Where I200 is the maximum peak of the diffraction at 2 θ of about 22.5 ° (200), and Iam is the diffraction intensity at 2 θ of about 18.5 ° (lowest portion between 002 plane and 110 plane).
(2) Transmission electron microscopy analysis: the microscopic morphology of the cellulose raw material and the regenerated cellulose sample was obtained by transmission electron microscopy (H-760). The cellulose raw material and the regenerated cellulose sample were diluted to a concentration of 0.01 wt% and sonicated at 45kHz and 180W for 10 min. Then respectively fishing out a drop of sample by using a special carbon film for TEM (transmission electron microscope), airing the sample at room temperature, and then dropwise adding a drop of 2 wt% uranyl acetate solution on the aired sample for dyeing for 30 min. And (4) sucking the excessive uranyl acetate dye solution by using absorbent paper, and then continuing to air-dry for 2 hours at normal temperature.
In summary, the invention provides a simple and efficient preparation method of the cellulose nano material, and cellulose nanocrystalline, cellulose nanosphere and cellulose nanofibril products with different appearances can be obtained according to different preparation processes, so that the requirements of industrial development on nanocelluloses with different aggregation state structures are met. In addition, the preparation method provided by the invention has the advantages that the reaction process is easy to regulate and control, the reaction system is single and stable, the regenerated solvent and the molten salt hydrate can be recycled, the production cost is further reduced, and the technical support is provided for large-scale production. The invention utilizes the molten salt system as the cellulose solvent to prepare the nano-cellulose material, enriches the nano-cellulose material products in different aggregation states and certainly promotes the marketization development of the cellulose nano-material.
Claims (10)
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.
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