Preparation method of high dispersibility corn cellulose nanocrystal

Abstract

This invention discloses a method for preparing highly dispersible corn cellulose nanocrystals, belonging to the technical field of cellulose nanocrystals. The method includes the following steps: organic acid pre-swelling and activation, dilute sulfuric acid hydrolysis, high-pressure microfluidic homogenization, dialysis, and concentration. The dilute sulfuric acid hydrolysis method involves adding ferric chloride and xylitol to a dilute sulfuric acid solution, stirring to dissolve, preheating to 70-85℃ and maintaining the temperature, adding activated cellulose filter cake, stirring thoroughly to disperse it evenly, reacting at a constant temperature of 70-85℃ for 10-15 minutes, and then adding 4-5 times the volume of deionized water to terminate the hydrolysis, obtaining a hydrolyzed suspension. This invention significantly shortens the hydrolysis time, ensures the product's white color and high crystallinity, improves the product's thermal stability, dispersion stability, and redispersibility, meeting the needs of high-end applications.

Description

Technical Field

[0001] This invention belongs to the technical field of cellulose nanocrystals, specifically relating to a method for preparing highly dispersible corn cellulose nanocrystals. Background Technology

[0002] Cellulose nanocrystals (CNCs) are rod-shaped nanomaterials extracted from natural cellulose. They possess high crystallinity, high specific surface area, excellent mechanical properties, and biodegradability, showing broad application prospects in fields such as composite material reinforcement, food packaging, and biomedicine.

[0003] Currently, the main methods for preparing cellulose nanocrystals are the concentrated sulfuric acid method and the organic acid method. Among them, the concentrated sulfuric acid method is the traditional mainstream method, which has a high hydrolysis efficiency, but it has obvious drawbacks: high-concentration concentrated sulfuric acid has strong corrosiveness and strong oxidizing properties, which not only places extremely high demands on equipment and easily causes equipment corrosion, but also leads to excessive destruction of the cellulose crystallization zone, resulting in a significant decrease in the thermal stability of the product; at the same time, after hydrolysis with concentrated sulfuric acid, repeated centrifugation, washing, and alkali neutralization are required, generating a large amount of high-salt wastewater, which has a large environmental impact, and the product surface has a high degree of sulfonation, making it prone to agglomeration after freeze-drying and having poor redispersibility, which limits its high-end applications.

[0004] Organic acid hydrolysis, as a green alternative, has the advantages of low corrosivity and environmental friendliness. However, it has the inherent defect of extremely low hydrolysis efficiency. It usually requires high concentrations of organic acid and high temperature conditions to react for several hours or even tens of hours to achieve effective degradation of the amorphous region of cellulose. The production efficiency is difficult to meet the needs of industrialization. Moreover, simple organic acid hydrolysis cannot form a stable charge anchoring system. The product surface has insufficient carboxyl content, low absolute value of Zeta potential, poor suspension stability, easy to cause stratification and precipitation, and the crystallinity of the product is also difficult to guarantee. The overall performance is far from meeting the requirements of practical applications.

[0005] Furthermore, existing technologies, whether using concentrated sulfuric acid or organic acid methods, suffer from low product yields, a tendency to yellow, and poor redispersibility. They also struggle to achieve a balance between efficient hydrolysis, high product performance, and environmental friendliness. Therefore, developing a method for preparing corn cellulose nanocrystals that is highly efficient, environmentally friendly, and produces nanocrystals with good thermal stability, excellent dispersibility, and a pure white color has become a pressing technical challenge in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for preparing highly dispersible corn cellulose nanocrystals. This method aims to solve the technical problems of low hydrolysis efficiency, poor product thermal stability, insufficient dispersibility, high environmental impact, and easy yellowing of the product color. It significantly shortens the hydrolysis time, ensures the product's white color and high crystallinity, and improves the product's thermal stability, dispersion stability, and redispersibility, thus meeting the needs of high-end applications.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for preparing highly dispersible corn cellulose nanocrystals includes the following steps: organic acid pre-swelling and activation, dilute sulfuric acid hydrolysis, high-pressure microfluidic homogenization, dialysis and concentration; (1) Organic acid pre-swelling and activation: Prepare an aqueous solution of organic acid with a mass concentration of 40-60%, add 0.5-1.0wt% solid acid catalyst to it, stir to disperse the catalyst evenly, add corn cellulose, control the bath ratio to 1:10-20, stir and pretreat for 50-70 min at 40-60℃ and 150-250rpm, after the reaction is completed, filter by plate and frame press to obtain activated cellulose filter cake, the filter cake does not need to be washed with water; The corn cellulose is derived from either corn stalks or corn cobs; The corn cellulose has a cellulose content of 90-95% and a crystallinity of 60-70%. The organic acid is one of citric acid and malic acid; The solid acid catalyst is a sulfonic acid-based mesoporous silica with a sulfonic acid loading of 1.5-2.0 mmol / g, a mesoporous pore size of 5-10 nm, and a particle size of 1-3 μm. The activated cellulose filter cake has a moisture content of 40-60%.

[0008] (2) Hydrolysis with dilute sulfuric acid: Add ferric chloride and xylitol to the dilute sulfuric acid solution, stir to dissolve, preheat to 70-85℃ and keep the temperature constant, add activated cellulose filter cake, stir thoroughly to disperse it evenly, react at a constant temperature of 70-85℃ for 10-15 minutes, then add 4-5 times the volume of deionized water to stop the hydrolysis and obtain a hydrolyzed suspension. The mass concentration of the dilute sulfuric acid solution is 8-12%; The amount of ferric chloride added to the dilute sulfuric acid solution is 0.2-0.3 wt% of the dilute sulfuric acid solution; The amount of xylitol added to the dilute sulfuric acid solution is 0.8-1.2 wt%. The mass ratio of the activated cellulose filter cake to the dilute sulfuric acid solution is 1:15-25; (3) High-pressure micro-jet homogenization: After microfiltration through a ceramic membrane, the hydrolyzed suspension is directly homogenized by circulation at a pressure of 80-120MPa 2-4 times without neutralization or washing; The ceramic membrane microfiltration has a pore size of 0.18-0.22 μm.

[0009] (4) Dialysis and concentration: Dilute sulfuric acid aqueous solution is used as dialysis fluid. The homogenized suspension is dialyzed for 30-40 hours to remove free acid and small molecule sugar. 1-2 hours before the end of dialysis, phytic acid is added to the dialysis system first, followed by sodium bicarbonate to generate soluble phytate. At the same time, the carbon dioxide bubbles generated by sodium bicarbonate help with dispersion. Then, the suspension is concentrated by rotary evaporation at 45-55℃ to obtain a highly dispersible corn cellulose nanocrystal suspension. The concentration of the dilute sulfuric acid solution is 0.004-0.006 mol / L; The molecular weight cutoff for dialysis is 8000-12000 Da; The amounts of phytic acid and sodium bicarbonate added are 0.012-0.018 wt% and 0.003-0.005 wt% of the homogenized suspension, respectively. The concentration of the highly dispersed corn cellulose nanocrystal suspension is 3-5 wt%.

[0010] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) In this invention, a 40-60% citric acid or malic acid solution is used to pre-swell and activate corn cellulose. Organic acid molecules can penetrate into the interior of cellulose, destroy the hydrogen bond network between cellulose molecules, and fully expose the amorphous regions of cellulose. At the same time, under the synergistic effect of the sulfonic acid-based mesoporous silica solid acid catalyst, the activation energy of the cellulose molecular chain is further reduced. In the subsequent 8-12% dilute sulfuric acid hydrolysis stage, the added FeCl3 acts as a Lewis acid catalyst, which can accelerate the breaking effect of dilute sulfuric acid on the amorphous regions. Xylitol is a non-reducing sugar alcohol and has unique chemical stability under dilute sulfuric acid hydrolysis conditions—it does not degrade, does not generate furfural, does not form a crust, and does not turn yellow. It occupies the active sites on the surface of cellulose through hydrogen bonds, effectively blocking the adsorption of glucose generated by hydrolysis on the surface of cellulose, avoiding the formation of a glucose degradation crust, and keeping the surface of cellulose clean. + It can continuously penetrate into the interior of cellulose to achieve efficient and thorough degradation of amorphous regions;

[0011] Based on the above synergistic effect, the present invention only requires 50-70 minutes of organic acid pre-activation and 10-15 minutes of dilute sulfuric acid hydrolysis to complete the efficient hydrolysis of the amorphous region of cellulose. The efficiency is significantly better than that of the organic acid method alone (which requires 3-24 hours) and close to or even exceeds that of the traditional concentrated sulfuric acid method (which requires 30-240 minutes). The hydrolysis efficiency is improved by nearly an order of magnitude, while avoiding the strong corrosiveness of the concentrated sulfuric acid method and the low efficiency of the organic acid method.

[0012] (2) The initial thermal decomposition temperature of corn cellulose nanocrystals obtained by the method of the present invention reaches 295-308℃, which is much higher than the 238℃ of the traditional concentrated sulfuric acid method CNC and the 272℃ of the conventional organic acid method CNC. The improvement in thermal stability is due to two factors. On the one hand, the addition of xylitol can effectively reduce the damage to the cellulose crystal structure during hydrolysis, significantly improve the crystal integrity of CNC, and thus improve the thermal stability of the product. On the other hand, the phytic acid added during the dialysis stage reacts with sodium bicarbonate to generate soluble phytate. The multiple phosphate groups in the phytate molecule can react with trace amounts of Fe in the system. 3+ A stable complex is formed, which is firmly adsorbed onto the CNC surface. It works synergistically with carboxyl groups to enhance negative charge properties, reduce active sites, and significantly reduce the degradation rate at high temperatures, thereby significantly improving the thermal stability of the product and meeting the needs of high-end applications such as high-temperature processing of polymer materials.

[0013] (3) The absolute value of the Zeta potential of the corn cellulose nanocrystals obtained by the method of the present invention is 55.3-60.1mV, which is much higher than the 32.6mV of the conventional organic acid method CNC. The suspension showed no sedimentation after standing at 4℃ for 60 days and had good redispersibility after freeze-drying. This is mainly because the phytate generated during the dialysis stage and the carboxyl groups on the CNC surface can synergistically anchor and fix the charge on the CNC surface, effectively avoiding charge loss during dialysis and preventing CNC agglomeration through electrostatic repulsion. At the same time, the carbon dioxide bubbles generated by the reaction of sodium bicarbonate and phytic acid can further promote the dispersion of CNC in the system and reduce local agglomeration. In addition, the xylitol residue can reduce the hydrogen bonding on the CNC surface and further improve the redispersibility, thus completely solving the problems of poor stability, easy agglomeration after freeze-drying, and difficulty in redispersibility of CNC suspension obtained by the traditional method.

[0014] (4) The CNC yield obtained by this invention is as high as 76.5-80.6%, and the product is white. This is mainly due to the synergistic effect of organic acid pre-activation and FeCl3 catalysis, which improves the selectivity and efficiency of dilute sulfuric acid hydrolysis and reduces excessive degradation and side reactions of cellulose. The addition of xylitol can reduce the damage to the cellulose crystal structure during hydrolysis and avoid the yield loss caused by the destruction of the crystal zone. At the same time, the process does not require repeated washing and neutralization, which reduces the loss of CNC. In addition, xylitol can inhibit the degradation and crusting of glucose during hydrolysis and avoid the generation of colored byproducts such as hydroxymethylfurfural, so that the product is white and has no visible yellowing. The purity is significantly better than that of the traditional concentrated sulfuric acid method. Detailed Implementation

[0015] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention are now described.

[0016] Example 1 A method for preparing highly dispersible corn cellulose nanocrystals includes the following steps: (1) Take 10g of corn cellulose (derived from corn cob, cellulose content 92%, crystallinity 65%), prepare citric acid into a 50% aqueous solution, add 0.75wt% sulfonic acid-based mesoporous silica, stir to disperse the catalyst evenly, add the corn cellulose, control the bath ratio to 1:15, stir and pretreat for 60min at 50℃ and 200rpm, after the reaction is completed, filter by plate and frame press to obtain activated cellulose filter cake with a water content of 50%, the filter cake does not need to be washed with water; The sulfonic acid-based mesoporous silica has a sulfonic acid group loading of 1.8 mmol / g, a mesoporous pore size of 8 nm, and a particle size of 2 μm.

[0017] (2) Add FeCl3·6H2O and xylitol to the dilute sulfuric acid solution, stir to dissolve, preheat to 78°C and maintain constant temperature. The mass concentration of the above dilute sulfuric acid solution is 10%, the amount of FeCl3·6H2O added is 0.42wt% of the dilute sulfuric acid solution, and the amount of xylitol added is 1wt% of the dilute sulfuric acid solution. Add the activated cellulose filter cake from step (1) to the dilute sulfuric acid solution, control the mass ratio of the filter cake to the dilute sulfuric acid solution to be 1:20, stir thoroughly to disperse it evenly, react at a constant temperature of 78°C for 12 min, and then add 4.5 times the volume of deionized water to stop the hydrolysis and obtain the hydrolyzed suspension.

[0018] (3) After the hydrolyzed suspension is microfiltered through a ceramic membrane with a pore size of 0.20 μm, it is directly circulated and homogenized 3 times under a pressure of 100 MPa without neutralization or washing to obtain a homogenized suspension.

[0019] (4) Using a 0.005 mol / L dilute sulfuric acid aqueous solution as the dialysis fluid, the homogenized suspension was placed in a dialysis bag with a molecular weight cutoff of 10000 Da and dialyzed for 35 h to remove free acid and small molecule sugars. 1.5 hours before the end of dialysis, phytic acid with a mass of 0.015% of the mass of the homogenized suspension was added to the dialysis system, followed by sodium bicarbonate with a mass of 0.004% of the mass of the homogenized suspension, to generate soluble phytate. At the same time, the carbon dioxide bubbles generated by sodium bicarbonate helped to disperse the phytate. Then, the suspension was concentrated by rotary evaporation at 50 °C to obtain a highly dispersed corn cellulose nanocrystal suspension with a concentration of 4 wt%.

[0020] Highly dispersible corn cellulose nanocrystal suspension was freeze-dried to obtain corn cellulose nanocrystals.

[0021] Performance characterization: The Zeta potential of the corn cellulose nanocrystals is -58.2 mV; the average length is 185 nm, the average diameter is 8.4 nm, the aspect ratio is 22.0, and the crystallinity is 78.8%; the initial thermal decomposition temperature is 302℃, the surface sulfonic acid group content is 0.03 mmol / g (conductivity titration), and the surface carboxyl group content is 0.63 mmol / g; the product is white in color with no visible yellowing; the yield (based on raw cellulose) is 79.2%; the suspension showed no visible precipitation or stratification after standing at 4℃ for 60 days, indicating excellent dispersion stability; after lyophilization, the nanocrystals were redispersed with deionized water, and the redispersed particle size deviated from the homogenized particle size by 8%, indicating good redispersibility.

[0022] Example 2 A method for preparing highly dispersible corn cellulose nanocrystals includes the following steps: (1) Take 10g of corn cellulose (derived from corn stalks, with a cellulose content of 90% and a crystallinity of 60%), prepare a 40% aqueous solution of citric acid, add 0.5wt% of sulfonic acid-based mesoporous silica to it, stir to disperse the catalyst evenly, add the corn cellulose, control the bath ratio to 1:10, stir and pretreat for 70min at 40℃ and 150rpm, filter by plate and frame press after the reaction, and obtain activated cellulose filter cake with a moisture content of 40%. The filter cake does not need to be washed with water. The sulfonic acid-based mesoporous silica has a sulfonic acid group loading of 1.5 mmol / g, a mesoporous pore size of 5 nm, and a particle size of 1 μm.

[0023] (2) Add FeCl3·6H2O and xylitol to the dilute sulfuric acid solution, stir to dissolve, preheat to 70℃ and maintain constant temperature. The mass concentration of the above dilute sulfuric acid solution is 8%, the amount of FeCl3·6H2O added is 0.33wt% of the dilute sulfuric acid solution, and the amount of xylitol added is 0.8wt% of the dilute sulfuric acid solution. Add the activated cellulose filter cake from step (1) to the dilute sulfuric acid solution, control the mass ratio of the filter cake to the dilute sulfuric acid solution to be 1:15, stir thoroughly to make it evenly dispersed, react at a constant temperature of 70℃ for 15 min, and then add deionized water with a volume of 4 times that of the dilute sulfuric acid solution to terminate the hydrolysis and obtain the hydrolyzed suspension.

[0024] (3) After the hydrolyzed suspension is microfiltered through a ceramic membrane with a pore size of 0.18 μm, it is directly circulated and homogenized twice under a pressure of 80 MPa without neutralization or washing to obtain a homogenized suspension.

[0025] (4) Using a 0.004 mol / L dilute sulfuric acid aqueous solution as the dialysis fluid, the homogenized suspension was placed in a dialysis bag with a molecular weight cutoff of 8000 Da and dialyzed for 30 h to remove free acid and small molecule sugars. One hour before the end of dialysis, phytic acid of 0.012% of the mass of the homogenized suspension was added to the dialysis system, followed by sodium bicarbonate of 0.003% of the mass of the homogenized suspension, to generate soluble phytate. At the same time, the carbon dioxide bubbles generated by sodium bicarbonate helped to disperse the suspension. The suspension was then concentrated by rotary evaporation at 45 °C to obtain a highly dispersed corn cellulose nanocrystal suspension with a concentration of 3 wt%.

[0026] Highly dispersible corn cellulose nanocrystal suspension was freeze-dried to obtain corn cellulose nanocrystals.

[0027] Performance characterization: The Zeta potential of the corn cellulose nanocrystals was -55.3 mV; the average length was 172 nm, the average diameter was 7.8 nm, and the aspect ratio was 22.1; the crystallinity was 76.2%; the initial thermal decomposition temperature was 295℃; the surface sulfonic acid group content was 0.02 mmol / g (conductivity titration), and the surface carboxyl group content was 0.58 mmol / g; the product was white in color with no visible yellowing; the yield was 76.5%; the suspension showed no visible precipitation or stratification after standing at 4℃ for 60 days, indicating excellent dispersion stability; after lyophilization, the nanocrystals were redispersed with deionized water, and the redispersed particle size deviated from the homogenized particle size by 9%, indicating good redispersibility.

[0028] Example 3 A method for preparing highly dispersible corn cellulose nanocrystals includes the following steps: (1) Take 10g of corn cellulose (derived from corn cob, cellulose content 95%, crystallinity 70%), prepare malic acid into a 60% aqueous solution, add 1.0wt% sulfonic acid mesoporous silica to it, stir to disperse the catalyst evenly, add the corn cellulose above, control the bath ratio to 1:20, stir and pretreat for 50min at 60℃ and 250rpm, after the reaction is completed, filter by plate and frame press to obtain activated cellulose filter cake with a water content of 60%, the filter cake does not need to be washed with water; The sulfonic acid-based mesoporous silica has a sulfonic acid group loading of 2.0 mmol / g, a mesoporous pore size of 10 nm, and a particle size of 3 μm.

[0029] (2) Add FeCl3·6H2O and xylitol to the dilute sulfuric acid solution, stir to dissolve, preheat to 85℃ and maintain constant temperature. The mass concentration of the above dilute sulfuric acid solution is 12%, the amount of FeCl3·6H2O added is 0.50wt% of the dilute sulfuric acid solution, and the amount of xylitol added is 1.2wt% of the dilute sulfuric acid solution. Add the activated cellulose filter cake from step (1) to the dilute sulfuric acid solution, control the mass ratio of the filter cake to the dilute sulfuric acid solution to be 1:25, stir thoroughly to make it evenly dispersed, react at a constant temperature of 85℃ for 10 min, and then add 5 times the volume of deionized water to stop the hydrolysis and obtain the hydrolyzed suspension.

[0030] (3) After the hydrolyzed suspension is microfiltered through a ceramic membrane with a pore size of 0.22 μm, it is directly circulated and homogenized 4 times under a pressure of 120 MPa without neutralization or washing to obtain a homogenized suspension.

[0031] (4) Using a 0.006 mol / L dilute sulfuric acid aqueous solution as the dialysis fluid, the homogenized suspension was placed in a dialysis bag with a molecular weight cutoff of 12000 Da and dialyzed for 40 h to remove free acid and small molecule sugars. Two hours before the end of dialysis, phytic acid with a mass of 0.018% of the mass of the homogenized suspension was added to the dialysis system, followed by sodium bicarbonate with a mass of 0.005% of the mass of the homogenized suspension, to generate soluble phytate. At the same time, the carbon dioxide bubbles generated by sodium bicarbonate helped to disperse the suspension. The suspension was then concentrated by rotary evaporation at 55 °C to obtain a highly dispersed corn cellulose nanocrystal suspension with a concentration of 5 wt%.

[0032] Highly dispersible corn cellulose nanocrystal suspension was freeze-dried to obtain corn cellulose nanocrystals.

[0033] Performance characterization: The Zeta potential of corn cellulose nanocrystals was -60.1 mV; the average length was 198 nm, the average diameter was 9.1 nm, and the aspect ratio was 21.8; the crystallinity was 80.5%; the initial thermal decomposition temperature was 308℃; the surface sulfonic acid group content was 0.04 mmol / g (conductivity titration), and the surface carboxyl group content was 0.68 mmol / g; the product was white in color with no visible yellowing; the yield was 80.6%; the suspension showed no visible precipitation or stratification after standing at 4℃ for 60 days, indicating excellent dispersion stability; after lyophilization, the nanocrystals were redispersed with deionized water, and the redispersed particle size deviated by 7% from the homogenized particle size, indicating good redispersibility.

[0034] Comparative Example 1 (Pure Concentrated Sulfuric Acid Method) (1) Take 10g of purified corn cellulose (same batch as in Example 1, cellulose content 92%, crystallinity 65%), add 200mL of 64% concentrated sulfuric acid, and mechanically stir at 45℃ for 45min to hydrolyze; (2) After the reaction is complete, add 10 times the volume of ice water to stop the hydrolysis, and repeatedly centrifuge and wash until the pH of the supernatant is 4; (3) The precipitate after washing was redispersed in deionized water, the solid content was adjusted to 1.5 wt%, and homogenized three times under high pressure microfluidic flow at 100 MPa. (4) Dialyze with deionized water for 48 hours until pH 7, then concentrate to 4 wt% by rotary evaporation at 50°C to obtain a cellulose nanocrystal suspension.

[0035] Highly dispersible corn cellulose nanocrystal suspension was freeze-dried to obtain corn cellulose nanocrystals.

[0036] Performance characterization: The Zeta potential of corn cellulose nanocrystals was -58.1 mV; the average length was 275 nm, the average diameter was 10.2 nm, and the aspect ratio was 21.1; the crystallinity was 68.5%; the initial thermal decomposition temperature was 238 °C; the surface sulfonic acid group content was 0.47 mmol / g, and the surface carboxyl group content was 0.12 mmol / g; the product was yellowish-brown with obvious visible yellowing; the yield was 78.5%; after the suspension was left to stand at 4 °C for 60 days, there was no visible precipitation or stratification; after freeze-drying, the nanocrystals were redispersed with deionized water, and the redispersed particle size deviated by 35% from the homogenized particle size, indicating poor redispersibility. Comparative Example 1, hydrolyzed with 64% concentrated sulfuric acid for 45 minutes, achieved a higher absolute value of the Zeta potential and 60-day stability, but its thermal decomposition temperature was only 238℃, 57-70℃ lower than that of the present invention; its crystallinity was only 68.5%, 8-12 percentage points lower than that of the present invention; and the redispersibility deviation of the powder after freeze-drying reached 35%, rendering it unusable after drying. This comparative example demonstrates that the traditional sulfuric acid method cannot simultaneously achieve both thermal stability and redispersibility.

[0037] Comparative Example 2 (Pure Organic Acid Method) (1) Take 10g of purified corn cellulose (same batch as in Example 1), add 200mL of 80% citric acid aqueous solution, and hydrolyze in an oil bath at 110℃ for 8h with stirring; (2) After the reaction is complete, add 5 times the volume of deionized water to terminate the reaction, and repeatedly centrifuge and wash until the pH of the supernatant is 5; (3) The precipitate after washing was redispersed in deionized water, the solid content was adjusted to 1.5 wt%, and homogenized three times under high pressure microfluidic flow at 100 MPa. (4) Dialyze with deionized water for 48 hours, then concentrate to 4 wt% by rotary evaporation at 50°C to obtain a cellulose nanocrystal suspension.

[0038] Highly dispersible corn cellulose nanocrystal suspension was freeze-dried to obtain corn cellulose nanocrystals.

[0039] Performance characterization: The Zeta potential of the corn cellulose nanocrystals was -32.6 mV; the average length was 245 nm, the average diameter was 12.6 nm, and the aspect ratio was 19.4; the crystallinity was 66.7%; the initial thermal decomposition temperature was 272 °C; the surface sulfonic acid group content was 0.01 mmol / g, and the surface carboxyl group content was 0.35 mmol / g; the product color was slightly yellow with a slight visible yellowing; the yield was 68.1%; the suspension showed obvious stratification and precipitation after standing at 4 °C for 7 days, indicating poor dispersion stability, far inferior to Examples 1-3; after lyophilization, the nanocrystals were redispersed with deionized water, and the redispersed particle size deviated by 58% from the homogenized particle size, indicating extremely poor redispersibility; the reaction time was 8 h, and the production efficiency was far lower than that of Examples 1-3.

[0040] Comparative Example 2 involved hydrolysis with 80% citric acid at 110°C for 8 hours. Although the carboxyl content reached 0.35 mmol / g and the thermal decomposition temperature was 272°C, the reaction time was as long as 8 hours. This invention only requires 50-70 minutes of organic acid pre-swelling and activation, and 10-15 minutes of acid hydrolysis, shortening the time by 6-8 times. However, the crystallinity was only 66.8%, the Zeta potential was only -32.6 mV, the dispersibility was extremely poor (settling after 7 days), the redispersibility deviation was as high as 58%, and the product was yellowish. This comparative example demonstrates that the simple organic acid method cannot obtain high crystallinity and high dispersibility CNC in a short time. This invention, through the synergistic design of "organic acid activation + dilute sulfuric acid hydrolysis," solves the inherent defects of low efficiency and poor dispersibility of the organic acid method.

[0041] Comparative Example 3 The same raw materials, processes and parameters as in Example 1 were used, except that the addition of xylitol to the dilute sulfuric acid hydrolysate was omitted; the dialysis, homogenization and concentration steps were kept the same.

[0042] Performance characterization: The zeta potential of the corn cellulose nanocrystals was -57.8 mV; the average length was 183 nm, the average diameter was 8.5 nm, and the aspect ratio was 21.5; the crystallinity was 73.6%; the initial thermal decomposition temperature was 282 °C; the surface sulfonic acid group content was 0.03 mmol / g, and the surface carboxyl group content was 0.62 mmol / g; the product showed a visible yellowing; the yield was 53.4%; after the suspension was left to stand at 4 °C for 38 days, slight precipitation occurred without obvious stratification, and after standing for 60 days, the precipitation became heavier, and the dispersion stability was worse than that of Example 1; after lyophilization, the nanocrystals were redispersed with deionized water, and the redispersed particle size deviated by 16% from the homogenized particle size, indicating poor redispersibility and overall performance inferior to that of Example 1.

[0043] Compared with Example 1, Comparative Example 3 only omitted xylitol. As a result, the crystallinity of the product decreased from 78.8% to 73.6%, a decrease of 5.2 percentage points; the thermal decomposition temperature decreased from 302℃ to 282℃, a decrease of 20℃; the yield decreased from 79.2% to 53.4%; the product color changed from pure white to visibly yellow; the redispersion deviation increased from 8% to 16%, doubling; and the stability period decreased from 60 days to 38 days, shortening by 22 days.

[0044] The product color changes from pure white to a visibly yellowish tinge, and the yield decreases significantly. This is likely because glucose and small-molecule oligosaccharides produced in the early stages of hydrolysis are adsorbed onto the cellulose surface. Under dilute acid conditions and at 70-85℃, these oligosaccharides rapidly degrade and form a crust. The resulting hydrophobic, dense gel-like layer directly binds to H+. + It blocks the surface of cellulose, preventing the hydrolysis of the amorphous regions inside from continuing; in this acidic environment, glucose and small molecule oligosaccharides will also be rapidly dehydrated and degraded, generating yellow substances such as furfural and hydroxymethylfurfural, which makes the final product yellowish.

[0045] This comparative example demonstrates that xylitol, without affecting the selective hydrolysis of cellulose amorphous regions by dilute sulfuric acid, results in more uniform hydrolysis, narrower nanocrystal size distribution, and a pure white product without yellowing. This effect is likely due to xylitol being a non-reducing sugar alcohol; it does not degrade, generate furfural, form a crust, or yellow in 8-12% dilute sulfuric acid at 70-85℃. It occupies active sites on the cellulose surface through hydrogen bonds, preventing glucose adsorption and subsequent crust formation, thus keeping the cellulose surface clean. The H₂ in the system... + It can continuously penetrate into the interior of cellulose, breaking the glycosidic bonds in the amorphous region without interruption, avoiding the termination of hydrolysis due to crusting, and achieving complete and efficient degradation of the amorphous region.

[0046] Comparative Example 4 The same raw materials, processes and parameters as in Example 1 were used, except that the addition of phytic acid and sodium bicarbonate during the dialysis stage was omitted; the hydrolysis, homogenization and concentration steps were kept the same.

[0047] Performance characterization: The zeta potential of the corn cellulose nanocrystals was -45.8 mV; the average length was 186 nm, the average diameter was 8.4 nm, and the aspect ratio was 22.1; the crystallinity was 78.5%; the initial thermal decomposition temperature was 300 °C; the surface sulfonic acid group content was 0.03 mmol / g, and the surface carboxyl group content was 0.63 mmol / g; the product was white in color with no visible yellowing; the yield was 78.5%; the suspension showed stratification after standing at 4 °C for 28 days, and completely stratified after standing for 60 days, indicating that the dispersion stability was significantly worse than that of Example 1; after lyophilization, the nanocrystals were redispersed with deionized water, and the redispersed particle size deviated by 22% from the homogenized particle size, indicating poor redispersibility and that the overall performance was worse than that of Example 1.

[0048] Comparative Example 4, compared to Example 1, omitted the addition of phytic acid and sodium bicarbonate during the dialysis stage. The results showed that after dialysis, the Zeta potential increased from -58.2 mV to -46.8 mV, a decrease of 11.4 mV in absolute value; the stabilization period decreased from 60 days to 27 days, a reduction of 33 days; and the redispersion deviation increased from 8% to 22%, an increase of 14 percentage points. This comparative example demonstrates that the phytate formed by phytic acid and sodium bicarbonate in a weakly acidic dialysis system anchors the surface charge through complexation with carboxyl groups and Fe³⁺, which is key to maintaining long-term dispersion stability and redispersion. Its addition caused the charge of the CNC to increase rather than decrease after dialysis. Compared to Example 1, the yield of Comparative Example 4 (78.5%) was essentially the same, indicating that phytic acid and sodium bicarbonate primarily affect dispersion stability rather than hydrolysis efficiency.

[0049] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing highly dispersible corn cellulose nanocrystals, characterized in that, The method includes the following steps: organic acid pre-swelling and activation, dilute sulfuric acid hydrolysis, high-pressure microfluidic homogenization, dialysis and concentration; The method for pre-swelling and activating the organic acid is as follows: the organic acid is prepared into an aqueous solution with a mass concentration of 40-60%, and 0.5-1.0 wt% of a solid acid catalyst is added to it. After stirring to disperse the catalyst evenly, corn cellulose is added, and the bath ratio is controlled at 1:10-20. The mixture is stirred and pretreated for 50-70 minutes at 40-60℃ and 150-250 rpm. After the reaction is completed, the mixture is filtered by plate and frame pressing to obtain an activated cellulose filter cake. The filter cake does not need to be washed with water. The method for hydrolyzing dilute sulfuric acid is as follows: ferric chloride and xylitol are added to a dilute sulfuric acid solution, stirred and dissolved, preheated to 70-85℃ and kept at a constant temperature, activated cellulose filter cake is added, stirred thoroughly to disperse it evenly, and reacted at a constant temperature of 70-85℃ for 10-15 minutes. Then, deionized water with a volume of 4-5 times that of the dilute sulfuric acid solution is added to terminate the hydrolysis, resulting in a hydrolyzed suspension.

2. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 1, characterized in that, In the steps of pre-swelling and activation of the organic acid The corn cellulose has a cellulose content of 90-95% and a crystallinity of 60-70%. The organic acid is one of citric acid and malic acid; The solid acid catalyst is a sulfonic acid-based mesoporous silica with a sulfonic acid loading of 1.5-2.0 mmol / g, a mesoporous pore size of 5-10 nm, and a particle size of 1-3 μm.

3. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 1, characterized in that, In the steps of pre-swelling and activation of the organic acid The activated cellulose filter cake has a moisture content of 40-60%.

4. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 1, characterized in that, In the step of dilute sulfuric acid hydrolysis, the mass concentration of the dilute sulfuric acid solution is 8-12%.

5. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 1, characterized in that, In the dilute sulfuric acid hydrolysis step The amount of ferric chloride added to the dilute sulfuric acid solution is 0.2-0.3 wt% of the dilute sulfuric acid solution; The amount of xylitol added to the dilute sulfuric acid solution is 0.8-1.2 wt%.

6. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 1, characterized in that, In the dilute sulfuric acid hydrolysis step, the mass ratio of the activated cellulose filter cake to the dilute sulfuric acid solution is 1:15-25.

7. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 1, characterized in that, The high-pressure microjet homogenization method involves circulating and homogenizing the hydrolyzed suspension 2-4 times under a pressure of 80-120MPa after microfiltration through a ceramic membrane without neutralization or washing. The ceramic membrane microfiltration has a pore size of 0.18-0.22 μm.

8. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 1, characterized in that, The dialysis and concentration method is as follows: using dilute sulfuric acid aqueous solution as dialysis fluid, the homogenized suspension is dialyzed for 30-40 hours to remove free acid and small molecule sugars. 1-2 hours before the end of dialysis, phytic acid is added to the dialysis system first, followed by sodium bicarbonate to generate soluble phytate. At the same time, the carbon dioxide bubbles generated by sodium bicarbonate help with dispersion. Subsequently, the suspension is concentrated by rotary evaporation at 45-55℃ to obtain a highly dispersible corn cellulose nanocrystal suspension.

9. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 8, characterized in that, In the dialysis and concentration steps The concentration of the dilute sulfuric acid solution is 0.004-0.006 mol / L; The molecular weight cutoff for dialysis is 8000-12000 Da.

10. The method for preparing highly dispersible corn cellulose nanocrystals according to claim 8, characterized in that, In the dialysis and concentration steps, the amounts of phytic acid and sodium bicarbonate added are 0.012-0.018 wt% and 0.003-0.005 wt% of the homogenized suspension, respectively.