A method for preparing metal salt-mediated hydrophilic cellulose nanosheets

Abstract

The application discloses a metal salt mediated hydrophilic cellulose nanosheet preparation method, and specific steps are as follows: firstly, the water content of cellulose material is dried to below 10%, and then the cellulose material is crushed and sieved; the sieved cellulose material, a hydrophilic material and a stainless steel ball are placed in a stainless steel ball mill tank, the rotation speed and time of the ball mill are adjusted, the ball mill is taken out after the ball mill is finished, and ball material separation is carried out to obtain the ball milled cellulose material; the ball milled cellulose material is heat treated through temperature programming; the heat treated cellulose material is mixed with water and then is statically placed for a long time, and the cellulose material is dispersed into fine particles in water and is suspended in water without precipitation, and the cellulose material has good hydrophilicity. The application has the following advantages and beneficial effects: the hydrogen bond network is broken by adding metal salt anions, the dehydration and carbonization of glucose pyran ring in the heat treatment process are inhibited by metal salt cations, and the cellulose chain agglomeration is prevented by bonding the phosphorus compound with the cellulose hydroxyl group.

Description

Technical Field

[0001] This invention relates to the field of natural polymer materials technology. Specifically, it describes a method for preparing hydrophilic cellulose nanosheets using a combination of mechanical and thermal energy. Background Technology

[0002] To achieve my country's carbon neutrality commitment, it is essential to restructure the energy structure from high-carbon to low-carbon and green. Biomass is the only renewable carbon-containing resource in nature and a carbon-neutral carrier. Therefore, innovative, efficient, green, and economical development of biomass resources plays a crucial and irreplaceable role in achieving carbon neutrality, and the biomass energy industry will usher in significant development opportunities. Cellulose is a widely available non-food biomass resource in nature. Through chemical and biological conversion, it can be used to produce energy chemicals and high-value-added materials, possessing enormous potential to replace fossil resources. However, due to its complex supramolecular structure, cellulose is difficult to melt and insoluble in water and common organic solvents, making processing extremely difficult and severely limiting the effective utilization of cellulose resources. Therefore, improving the dispersibility of cellulose in different solvents through nano-processing and surface modification is currently a hot topic in cellulose research and utilization.

[0003] Nanocellulose (cellulose with any one dimension reduced to within 100 nm) has attracted great attention from scientists due to its excellent physical and chemical properties. The preparation methods for nanocellulose mainly include chemical methods, mechanical methods, enzymatic methods, or a combination of both. Acid hydrolysis removes the amorphous regions of the fiber, leaving tightly packed and orderly crystalline parts, resulting in cellulose nanocrystals with high crystallinity and morphology. However, the suspension has poor stability, is prone to aggregation, and generates waste acid that pollutes the environment and corrodes equipment. Mechanical methods for preparing nanocellulose are relatively environmentally friendly and simple: for example, invention patent 201510812157.9 discloses a method for preparing cellulose nanosheets. First, cellulose material is mixed and ground with a hydrophobic material for 1-48 hours, causing the cellulose to gradually separate into sheets. Then, ultrasonic dispersion and separation remove the hydrophobic material, resulting in hydrophobic cellulose nanosheets with a smooth surface and polydisperse lateral dimensions and morphology. Invention patent 201610114924.3 reports a method for preparing superhydrophobic nanopaper, which involves first oxidizing cellulose with TEMPO, then homogenizing under high pressure to obtain nanocellulose, and finally filtering to form a film. This preparation process requires precise control of the material ratios and is complex. Invention patent 201910279739.3 reports a method for preparing hydrophobically modified cellulose nanomaterials, which involves grinding cellulose and hydrophobic materials in a mixed additive for 4-48 hours. The mixed additive is a mixture of polar and non-polar solvents, and the ratio of cellulose to the mixed additive is 1-2 g: 40 mL. After grinding, filtration yields hydrophobic cellulose nanofibers and cellulose nanosheets. This preparation process is time-consuming and requires a large amount of solvent. Existing mechanical cellulose nanoprocessing technologies mainly focus on how to obtain hydrophobic nanocellulose or prepare hydrophobic cellulose composite materials. However, the hydrophilicity and hydrophobicity of nanocellulose mainly depend on the degree of exposure of its surface hydroxyl groups. Therefore, hydrophilic nanocellulose has more exposed hydroxyl groups on its surface. These exposed hydrophilic hydroxyl groups can be replaced by different functional groups through chemical reactions, which is beneficial for its deep processing to obtain more high-value-added materials and energy chemicals.

[0004] Although significant progress has been made in cellulose nanocrystals and hydrophobic cellulose nanosheets, detailed reports on hydrophilic two-dimensional cellulose materials have yet to be published. We successfully prepared hydrophilic two-dimensional cellulose nanosheets by modulating the synergistic effects of external environmental factors (force and heat) and polar media, laying the foundation for the development of cellulose two-dimensional nanomaterials. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing hydrophilic cellulose nanosheets using a combination of mechanical and thermal energy. To achieve this objective, the invention employs the following technical solution: First, the water content of the cellulose material is dried to below 10%, then pulverized and sieved. The sieved cellulose material, along with a hydrophilic material and stainless steel balls, is placed in a stainless steel ball mill jar. The speed and time of the ball mill are adjusted. After milling, the material is removed and separated from the ball material to obtain the milled cellulose material. The milled cellulose material is then heat-treated using a programmed temperature control system. After heat treatment, the cellulose material is mixed with water and allowed to stand for an extended period. It disperses into fine particles that remain suspended in the water without settling, exhibiting excellent hydrophilicity. Both the milling and heat treatment processes of the cellulose material and the hydrophilic material are carried out under solvent-free conditions.

[0006] The cellulose material obtained through ball milling and heat treatment mainly consists of cellulose nanosheets, accounting for 60-90 wt% (preferably 70-80 wt%) of the total cellulose content. The remainder consists of irregularly shaped sub-nanometer and micron-sized particles.

[0007] The cellulose nanosheets described herein exhibit polydisperse irregular shapes (serrated, funnel-shaped, triangular, leaf-shaped, etc.) with an area ranging from 0.1 to 200 μm. 2 (Preferred size: 20-150µm) 2 The thickness is 0.1-8nm (preferably 1-5nm).

[0008] The hydrophilic material includes one or more mixtures of phosphorus-containing compounds and metal salts.

[0009] The phosphorus-containing compound includes one or a mixture of several of the following: phosphorus pentoxide, phosphoric acid, pyrophosphate, solid phosphoric acid, and phosphate esters (preferably phosphorus pentoxide and phosphoric acid, most preferably phosphorus pentoxide).

[0010] The metal salts mentioned include one or a mixture of several of alkali metal halides and alkaline earth metal halides.

[0011] The alkali metal halide salt cation is Li + Na + K + 、Rb + Cs + (Preferred Na) + K + The most likely choice is K. + One or more of these, the anion being F - Cl - ,Br - (Preferred Br) - One or two or more of them.

[0012] The alkaline earth metal halide salt cation is Be. 2+ Mg 2+ Ca 2+ 、Sr 2+ Ba 2 (Preferred Be) 2+ Mg 2+ Ca 2+ The optimal choice is Mg 2+ One or two or more of them + The anion is F - Cl - ,Br - (Preferred Cl) - One or two or more of them.

[0013] The mass ratio of the phosphorus-containing compound to the cellulose material is 0.001 to 3:100 (equivalent to 0.001% to 3%) (preferably 0.5% to 2%, most preferably 1.0%).

[0014] The mass ratio of the metal salt to the cellulose material is 0.001 to 3:100 (equivalent to 0.001% to 3%) (preferably 0.01% to 1%, most preferably 0.4%).

[0015] The ball milling process is carried out in a planetary ball mill at a speed of 200-800 rpm (preferably 300-600 rpm, most preferably 500 rpm) for a duration of 1-30 min (5-20 min, most preferably 15 min).

[0016] The stainless steel ball has a diameter of 0.5mm-10mm (preferably 1.0mm-5mm, most preferably 2.0mm), and the mass ratio of cellulose material to stainless steel ball is 1:300-1:1 (preferably 1:100-1:10, most preferably 1:15).

[0017] The heat treatment temperature is 50-180℃ (preferably 100-160℃, most preferably 140℃), and the heat treatment time is 10-120min (preferably 20-90min, most preferably 60min).

[0018] The ball milling and heat treatment processes for both the fiber material and the hydrophilic material are carried out under solvent-free conditions.

[0019] The cellulose material is selected from microcrystalline cellulose, One or more cellulose-containing biomass materials from PH-101, α-cellulose, waste paper, waste cardboard boxes and other fiber pulp products.

[0020] The beneficial effects of this invention are as follows: By adding metal salt anions, the hydrogen bond network can be rapidly broken during the ball milling process, allowing hydrophilic material molecules to penetrate into the hydrophilic sheets of cellulose crystals. Meanwhile, the metal salt cations effectively inhibit the dehydration and carbonization of the glucose pyran ring during heat treatment. Phosphorus-containing compounds prevent the aggregation of cellulose chains by bonding with the cellulose hydroxyl groups. The cellulose sheets are rapidly exfoliated along the hydrophilic surface to form hydrophilic cellulose nanosheets, exhibiting good dispersibility and accessibility in polar solvents such as water. This method is simple, easy to implement, and energy-efficient, providing a sound scientific basis and technological foundation for the further processing and conversion of cellulose materials into chemicals and high-value-added materials. Attached image description:

[0021] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0022] Figure 1 Atomic force microscopy (AFM) images of cellulose nanosheets in Comparative Example 1 and their dispersibility in water.

[0023] Figure 2 Atomic force microscopy (AFM) of the cellulose material in Comparative Example 2.

[0024] Figure 3 Atomic force microscopy (AFM) of the cellulose material in Comparative Example 3.

[0025] Figure 4 Atomic force microscopy (AFM) image of the cellulose material in Example 1 and its dispersibility in water.

[0026] Figure 5 Atomic force microscopy (AFM) image of the cellulose material in Example 2 and its dispersibility in water. Detailed Implementation

[0027] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, will provide further details. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims and their equivalents. A Raman-Atomic Force Microscopy (AFM) imaging analysis system (JPK NanoWizard Ultra Speed ​​& RenishawinVia) is used, employing both tapping mode (primary) and contact mode (secondary) to scan the sample, obtaining elevation and phase maps. The sample is evenly sprinkled on a glass slide, suspended by a drop of water, and then dried under an alcohol lamp to allow the sample to adhere to the slide. Weakly adhered samples are then blown away with a syringe, allowing direct scanning of the sample's surface morphology under an atomic force microscope.

[0028] Comparative Example 1

[0029] First, microcrystalline cellulose ( PH-101 (particle size 50um) was dried until the water content was less than 10wt%, and then 10g of microcrystalline cellulose was taken ( The cellulose nanoparticles (pH-101, 50 μm) were dissolved in 100 mL of 0.1 M / L NaOH solution and stirred in an ice bath for 30 minutes to form a homogeneous phase. Then, 200 mL of water was added to precipitate the nanoparticles, which were then filtered and dried to obtain regenerated cellulose. The obtained powder was prepared into an ethanol suspension (solid-liquid mass ratio 0.05%), sonicated for 5 minutes, and 2 μL of the dispersion was coated onto a silicon wafer and allowed to dry naturally. The morphology and height were observed using an atomic force microscope, revealing that the cellulose nanoparticles all exhibited sheet-like structures approximately 4 nm thick. The obtained cellulose nanomaterials were mixed with water at a solid-liquid mass ratio of 20 wt% and stirred for 30 minutes, then allowed to stand for 10 minutes, resulting in rapid precipitation (e.g., precipitate). Figure 1 ).

[0030] Comparative Example 2

[0031] First, microcrystalline cellulose ( PH-101 (particle size 50um) was dried until the water content was less than 10wt%, and then 10g of microcrystalline cellulose was taken ( 0.05 g of phosphorus pentoxide, 0.02 g of potassium bromide, and 150 g of stainless steel balls were placed in a ball mill jar and ground at 500 rpm for 10 minutes. The resulting powder was then prepared into an ethanol suspension (solid-liquid mass ratio 0.05%), sonicated for 5 minutes, and 2 μL of the dispersion was coated onto a clean silicon wafer and allowed to dry naturally. The morphology and height were observed using an atomic force microscope. Cellulose mainly existed in the form of nano-sized particles (3-6 nm thick) (e.g., PH-101, 50 μm), 0.05 g of phosphorus pentoxide, 0.02 g of potassium bromide, and 150 g of stainless steel balls. Figure 2 The formation of cellulose nanoparticles is due to the fiber fragments of microcrystalline cellulose raw material being collided with grinding balls and subjected to shearing force to become small particles; the obtained cellulose nanomaterials are mixed with water at a solid-liquid mass ratio of 20wt% and stirred for 30 minutes, then left to stand for 30 minutes, and quickly precipitated.

[0032] Comparative Example 3

[0033] First, microcrystalline cellulose ( PH-101 (particle size 50um) was dried until the water content was less than 10wt%, and then 10g of microcrystalline cellulose was taken ( After a simple mixture of PH-101 (50 μm) and 0.02 g potassium bromide, the mixture was directly heated at 140 °C for 60 minutes. The resulting powder was then prepared into an ethanol suspension (solid-liquid mass ratio 0.05%), sonicated for 5 minutes, and 2 μL of the dispersion was coated onto a clean silicon wafer and allowed to dry naturally. The morphology and height were observed using an atomic force microscope. Cellulose was found to exist primarily as nano-sized particles (100-200 nm thick) (e.g., 50 μm). Figure 3 The formation of cellulose nanoparticles is due to the breakage of the original fibers of microcrystalline cellulose raw material under ultrasonic action; the obtained cellulose nanomaterials are mixed with water at a solid-liquid mass ratio of 20wt% and stirred for 30 minutes, then left to stand for 10 minutes, and quickly precipitated.

[0034] Example 1

[0035] First, microcrystalline cellulose ( PH-101 (particle size 50um) is dried until the water content is less than 10wt%, then 10g of the dried product is taken. PH-101, 0.05g phosphorus pentoxide, 0.02g potassium bromide, and a 150g stainless steel ball mill with a diameter of 2mm were placed in the grinding jar of a planetary ball mill and ground at 500rpm for 10 minutes. The ground sample was then heated at 130℃ for 50 minutes to obtain cellulose powder. After mixing with water and stirring for 10 minutes, a viscous porridge-like substance (solid-liquid mass ratio 15%) was formed. After standing for 60 minutes, almost no sediment was observed. The obtained powder was prepared into an ethanol suspension (solid-liquid mass ratio 0.05%), sonicated for 5 minutes, and 2µl of the dispersion was coated onto a silicon wafer and allowed to dry naturally. The morphology and height were observed using an atomic force microscope, revealing that the cellulose was approximately 4-5nm thick with an area of ​​0.1-20µm. 2 Mainly in sheet form (e.g.) Figure 4 It exists in an irregular polydisperse state (triangular, trapezoidal, leaf-shaped, etc.), accounting for 85 wt% of the total cellulose, with the remainder being irregular small particles formed by the re-aggregation of cellulose nanosheets.

[0036] Example 2

[0037] First, microcrystalline cellulose ( PH-101 (particle size 50um) is dried until the water content is less than 10wt%, then 10g of the dried product is taken. PH-101, 0.05g phosphorus pentoxide, 0.02g potassium bromide, and 150g stainless steel ball mill were placed in the grinding jar of a planetary ball mill and ground at 500rpm for 15 minutes. The ground sample was then heated at 140℃ for 60 minutes to obtain cellulose powder. After mixing with water and stirring for 10 minutes, a viscous porridge-like substance (solid-liquid mass ratio 20%) was formed. After standing for 60 minutes, almost all cellulose particles precipitated. The obtained powder was prepared into an ethanol suspension (solid-liquid mass ratio 0.05%), sonicated for 5 minutes, and 2µl of the dispersion was coated onto a silicon wafer and allowed to dry naturally. The morphology and height were observed using an atomic force microscope, revealing that the cellulose particles had a thickness of approximately 1-3nm and an area of ​​0.5-100µm. 2 Mainly in sheet form (e.g.) Figure 5 It exists in an irregular polydisperse state (funnel-shaped, irregular quadrilateral, serrated, etc.), accounting for 80 wt% of the total cellulose, with the remainder being irregular small particles formed by the re-aggregation of cellulose nanosheets.

[0038] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for preparing metal salt-mediated hydrophilic cellulose nanosheets, characterized in that, Includes the following steps: (a) First, dry the water content of the cellulose material to below 10 wt% and then pulverize it for later use; (b) Cellulose material, hydrophilic material, and stainless steel balls are placed in a stainless steel ball mill jar. The speed and time of the ball mill are adjusted. After the ball milling is completed, the material is removed and the ball and material are separated to obtain the ball-milled cellulose material. The hydrophilic material includes a mixture of phosphorus-containing compounds and metal salts. The ball milling process is carried out in a planetary ball mill at a speed of 300-600 rpm for 5-20 minutes. The diameter of the stainless steel balls is 0.5 mm-10 mm, and the mass ratio of cellulose material to stainless steel balls is 1:300-1:

1. (c) The ball-milled cellulose material is subjected to heat treatment; the heat treatment temperature is 50-180℃ and the heat treatment time is 10-120min; The cellulose material obtained by ball milling and heat treatment has cellulose nanosheets accounting for 60wt%-90wt% of the total cellulose content; the phosphorus-containing compound includes phosphorus pentoxide, phosphoric acid, pyrophosphate, solid phosphoric acid or a mixture thereof; the metal salt includes alkali metal halide salts and alkaline earth metal halide salts or a mixture thereof.

2. The preparation method according to claim 1, characterized in that: The cellulose nanosheets described herein have a polydisperse, irregular surface shape with an area ranging from 0.1 to 200 μm. 2 The thickness is 0.1-8 nm.

3. The preparation method according to claim 2, characterized in that: The cellulose nanosheets described herein have a polydisperse, irregular surface shape with an area of ​​20-150 μm. 2 The thickness is 1-5 nm.

4. The preparation method according to claim 1, characterized in that: The cellulose material obtained by ball milling and heat treatment has cellulose nanosheets accounting for 70wt%-80wt% of the total cellulose.

5. The preparation method according to claim 1, characterized in that: The phosphorus-containing compounds include one or two of phosphorus pentoxide and phosphoric acid.

6. The method according to claim 1, characterized in that: The alkali metal halide salt cation is Li + Na + K + 、Rb + Cs + One or more of them, the anion is F - Cl - ,Br - One or two or more of them; The alkaline earth metal halide salt cation is Be. 2+ Mg 2+ Ca 2+ 、Sr 2+ Ba 2 One or two or more of them + The anion is F. - Cl - ,Br - One or two or more of them.

7. The method according to claim 6, characterized in that: The alkali metal halide salt cation is Na. + K + One or two of them, the anion is Br - ; The alkaline earth metal halide salt cation is Be. 2+ Mg 2+ Ca 2+ One or two or more of them + The anion is Cl. - .

8. The preparation method according to claim 1, characterized in that: The phosphorus-containing compound and the cellulose material comprise 0.001 wt% to 3 wt% of the cellulose material by mass. The metal salt is 0.001 wt% to 3 wt% of the cellulose material.

9. The preparation method according to claim 8, characterized in that: The phosphorus-containing compound and the cellulose material comprise 0.5 wt% to 2 wt% of the cellulose material by mass. The metal salt is 0.01 wt% to 1 wt% of the cellulose material.

10. The preparation method according to claim 1, characterized in that: The ball milling process is carried out in a planetary ball mill at a speed of 300-600 rpm for 5-20 minutes; the stainless steel balls have a diameter of 1.0 mm-5 mm, and the mass ratio of cellulose material to stainless steel balls is 1:100-1:

10.

11. The preparation method according to claim 1, characterized in that: The heat treatment temperature is 100-160℃, and the heat treatment time is 20-90min.

12. The preparation method according to claim 1, characterized in that: The ball milling and heat treatment processes for the cellulose and hydrophilic materials were both carried out under solvent-free conditions. The cellulose material is selected from one or more cellulose-containing biomass materials selected from microcrystalline cellulose, Avicel® PH-101, α-cellulose, waste paper, and waste cardboard boxes.

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