Method for realizing surface esterification of cellulose in aqueous solution

Abstract

The present invention belongs to the technical field of cellulose modification, and particularly relates to a method for realizing surface esterification of cellulose in an aqueous solution. The method uses an organic alkali catalyst and a vinyl ester esterification agent to prepare a cellulose surface esterification product by means of a one-pot method in an aqueous solution. The method is simple, rapid and easy to implement, and solves the problem of incapability of directly esterifying cellulose in aqueous solutions, thus enabling direct surface esterification of cellulose without the need of drying or solvent replacement; moreover, solutions following the reaction can be directly and cyclically utilized, thus significantly increasing the efficiency.

Description

A method for realizing cellulose surface esterification in aqueous solution Technical Field

[0001] The invention belongs to the technical field of cellulose modification, and particularly relates to a method for realizing cellulose surface esterification in an aqueous solution. Background Art

[0002] Cellulose is a natural polysaccharide compound composed of glucose molecules connected by β-1,4-glycosidic bonds to form a long chain structure. It is widely present in plant cell walls and accounts for more than 50% of the carbon content in the plant kingdom. It provides structural support and stability to plants and has good renewability and biodegradability. In industry, cellulose can be used to make paper, textiles, food additives, biofuels, etc. In scientific research, cellulose is used as a biomedical material and tissue engineering scaffold. However, cellulose has some limitations in its natural form, such as hydrophilicity, low mechanical strength, and easy agglomeration. To overcome these limitations and expand its application areas, cellulose esterification modification has been widely studied and applied.

[0003] In particular, surface esterification of cellulose can improve its hydrophobic properties without altering its internal structure, effectively increasing its compatibility with hydrophobic substrates. Side chain structures can also be introduced to enhance the interaction between cellulose molecules, thereby improving the mechanical strength and durability of cellulose materials. The introduction of different functional groups can also impart new chemical properties and functions to cellulose.

[0004] Cellulose esterification is a chemical reaction in which cellulose reacts with an acid anhydride (anhydride compound or anhydrifying agent) to form a cellulose ester. This reaction is achieved by reacting the anhydride group in the anhydride with the hydroxyl group in the cellulose to form an ester bond. Commonly used anhydride compounds in cellulose esterification include inorganic anhydrides (such as acid chloride and anhydride chloride) and organic anhydrides (such as acetic anhydride and pyruvic anhydride). Anhydrifying agents such as N,N-dimethylacetamide and N-bromosuccinimide (DMAc / LiBr) can also be used to catalyze the cellulose esterification reaction.

[0005] Current cellulose esterification research primarily involves using dry cellulose materials in organic solvents. Cetin, NS. et al. conducted an ester exchange reaction using vinyl acetate in the organic solvent DMF (Cetin, NS. et al., Macromol. Biosci. 2009, 9, 997-1003). However, organic solvents commonly used in cellulose esterification, such as dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF), can evaporate into the atmosphere and pollute the environment. Their high cost limits the large-scale application of cellulose esterification. Furthermore, the recovery and disposal of waste solvents also present technical and cost challenges. Tripathi et al. used dried fiber samples, then exchanged solvents with acetone, added a mixed solvent of toluene, acetic acid, and sulfuric acid, and finally added acetic anhydride to react and produce esterified cellulose (Anurodh Tripathi, Mariko Ago. et al., ACS applied materials & interfaces. 2018, 10, 51: 44776-44786.). The drying process requires removing a large amount of water from the cellulose, which causes irreversible damage to the chemical bonds between the original fibers. This phenomenon is also called "keratinization," which ultimately reduces the dispersibility, reactivity, and performance of the cellulose in applications.

[0006] Therefore, developing a method for cellulose surface esterification that does not require an organic solvent and whose reaction conditions are more suitable for industrial production has become a technical problem that needs to be urgently solved by those skilled in the art.

[0007] Summary of the Invention

[0008] In view of this, the first object of the present invention is to address the problems existing in the prior art and provide a cellulose surface esterification method that does not require an organic solvent and whose reaction conditions are more suitable for industrial production. The method has mild reaction conditions and the reaction system can be used for subsequent production applications, which promotes the development of cellulose esterification modification in a simple, environmentally friendly, green and sustainable direction.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A method for achieving cellulose surface esterification in an aqueous solution comprises the following steps:

[0011] (I) adding a cellulose raw material to an aqueous solution to make the mixed solution concentration be 0.1% to 30% by mass, and stirring uniformly;

[0012] (II) adding an organic base catalyst and a vinyl ester esterifying agent to the mixed solution obtained in step (I), stirring vigorously, reacting at a constant temperature for a period of time, and allowing the mixture to stand after the reaction, filtering, and drying to obtain a cellulose surface esterification product;

[0013] (III) After adding a regeneration agent to the liquid obtained by filtration in step (II), the liquid is recycled for subsequent cellulose surface esterification reactions.

[0014] It is worth noting that, although the prior art has disclosed a technical solution for preparing surface-esterified microcrystalline cellulose using an esterification method of mixed acid in an aqueous solution, the temperature required for the reaction is high, and the reaction time is 3-20 times that of the present invention at the same reaction temperature. In addition, the use of mixed acid results in the need to neutralize the product in post-processing, and the post-processing method of the reaction system is not mentioned. From a principle point of view, surface esterification of microcrystalline cellulose in mixed acid is essentially a reaction in which carboxylic acid and hydroxyl esters lose water, and therefore, the reactivity of carboxylic acid and hydroxyl is much lower than that of anhydride. The present invention adopts vinyl esters as esterifying agents. Under the action of an alkaline catalyst, vinyl esters and cellulose hydroxyl groups form cellulose esters and vinyl alcohol through ester exchange. Vinyl alcohol is structurally unstable and will be immediately converted into acetaldehyde once generated, thereby accelerating the reaction forward and achieving rapid esterification of the cellulose surface.

[0015] Compared with the existing technology, the present invention uses an organic base catalyst and a vinyl ester esterification agent to prepare a cellulose surface esterification product in an aqueous solution in a one-pot method. This method does not require drying of the cellulose raw material, avoids the internal structure transformation caused by drying of the raw material, and improves the reaction efficiency; and the reaction system does not require an organic solvent, which reduces environmental hazards, reduces reaction costs, and lowers the operating threshold.

[0016] Furthermore, the cellulose in step (I) includes one or more of pulp, cotton, microcrystalline cellulose, nanocellulose, and wood fiber.

[0017] Furthermore, the organic base catalyst in step (II) includes at least one of 4-dimethylaminopyridine, monoethanolamine, diethanolamine, triethanolamine, monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, and triethylamine, and the amount of the organic base catalyst added is 1 to 100% of the mass of the cellulose raw material.

[0018] Furthermore, the vinyl ester esterifying agent in step (II) includes one or more of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl hexanoate, vinyl octanoate, vinyl caprate, and vinyl laurate, and the vinyl ester esterifying agent.

[0019] Furthermore, the reaction temperature of step (II) is 0°C-70°C, and the reaction time is 1 min-5 h.

[0020] It is worth noting that the reaction temperature of the present invention is set to room temperature, and no heating is required, which reduces the energy consumption and risk level of the reaction and facilitates the scale-up and industrial production of the reaction system.

[0021] Furthermore, step (III) includes detecting, calibrating, and filtering the liquid to obtain the vinyl ester esterifying agent and organic base catalyst content, and replenishing the cellulose raw material or a cellulose raw material mixture, the vinyl ester esterifying agent, the organic base catalyst, and a regeneration reagent so that the concentration of the cellulose raw material mixture in the reaction solution is 0.1% to 30% by mass, the mass of the organic base catalyst is not less than 1% by mass of the cellulose raw material, and the mass of the vinyl ester esterifying agent is not less than 5% by mass of the cellulose raw material. Furthermore, the regeneration reagent includes at least one of sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, calcium hydroxide, and ammonia water, and the amount of the regeneration reagent added is not greater than the initial molar amount of the organic base catalyst.

[0022] It is worth noting that esterification, as a reversible reaction, cannot achieve complete reaction of the reaction raw materials, and the concentration ratio of reactants to products in the reaction system has a significant impact on the reaction rate and reaction efficiency of the esterification reaction. Therefore, in industrial production, considering both production costs and production efficiency, it is necessary to control the concentration of reactants in the reaction system. The present invention, through the recycling of the reaction solution, avoids the waste of esterification reaction raw materials on the one hand, and simplifies the post-reaction treatment procedure on the other hand, providing a more environmentally friendly solution for the industrial development of cellulose surface esterification.

[0023] Compared with the prior art, the present invention has the following significant advantages:

[0024] (1) The present invention discloses a method for preparing cellulose surface esterification products in an aqueous solution in a one-pot process using an organic base catalyst and a vinyl ester esterifying agent, so that the raw materials do not need to be dried and can react in an aqueous system, thereby avoiding the internal structure transformation caused by the drying of the raw materials and improving the reaction efficiency; and no organic solvent is required as the reaction medium, thereby reducing the harm to the environment and greatly reducing the reaction cost and operation difficulty.

[0025] (2) The reaction temperature can be carried out at room temperature without heating, which reduces energy consumption and greatly reduces the degree of danger; organic base catalysts and vinyl ester esterification agents can be reused and recycled, reducing raw material costs, which is in line with the current "carbon neutrality" and sustainable development strategy.

[0026] (3) The preparation process proposed in the present invention is simple to operate, has a rapid reaction, and is low in cost. It does not require complex reaction equipment and is of great benefit to industrial-scale production, thereby enhancing the practical application potential of modified cellulose. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative work.

[0028] FIG1 is the FT-IR spectrum of the modified cellulose measured in the present invention. DETAILED DESCRIPTION

[0029] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0030] The term "embodiment" is used herein specifically to describe any embodiment as "exemplary," and should not be construed as superior or preferable to other embodiments. Performance indicators in the embodiments of this application were tested using conventional testing methods in the art, unless otherwise specified. It should be understood that the terms used in this application are intended solely to describe specific implementations and are not intended to limit the disclosure herein.

[0031] Unless otherwise specified, the technical and scientific terms used herein have the same meanings as commonly understood by ordinary technicians in the technical field to which this application belongs; other experimental methods and technical means not specifically specified in this application refer to experimental methods and technical means commonly used by ordinary technicians in this field.

[0032] In order to better illustrate the content of the present application, numerous specific details are provided in the specific examples below. It should be understood by those skilled in the art that without certain specific details, the present application can be implemented equally. In the embodiments, some methods, means, instruments, equipment etc. well known to those skilled in the art are not described in detail, so as to highlight the purport of the present application. Under the premise of not conflicting, the technical features disclosed in the embodiments of the present application can be combined in any way, and the technical scheme obtained belongs to the content disclosed in the embodiments of the present application.

[0033] For a better understanding of the present invention, the present invention is further specifically described below through the following examples, but it should not be understood as limiting the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above invention content are also considered to fall within the scope of protection of the present invention.

[0034] Example 1

[0035] (1) Add 50 mL of water, 2 g of microcrystalline cellulose, 1.0 g of 4-dimethylaminopyridine (DMAP), and 20 mL of vinyl acetate into a 100 mL conical flask.

[0036] (2) Stopper the conical flask and stir vigorously at room temperature for 1 hour. After the reaction is complete, allow to stand, filter, wash the solid, and dry to obtain surface-esterified cellulose. Infrared spectroscopy was used to determine that the surface of the surface-modified cellulose contained a large number of ester groups (Figure 1).

[0037] (3) After adding 1.0 g of potassium carbonate to the filtered liquid for regeneration, the liquid can be recycled to continue the cellulose surface esterification reaction.

[0038] Example 2

[0039] (1) Add 50 mL of water, 2 g of microcrystalline cellulose, 0.2 g of 4-dimethylaminopyridine (DMAP), and 20 mL of vinyl acetate into a 100 mL conical flask.

[0040] (2) Stopper the conical flask and stir vigorously at room temperature for 1 hour. After the reaction is complete, allow to stand, filter, wash the solid, and dry to obtain surface-esterified cellulose. Infrared spectroscopy was used to determine that the surface of the surface-modified cellulose contained a large number of ester groups (Figure 1).

[0041] (3) After adding 0.1 g of sodium carbonate to the filtered liquid for regeneration, the liquid can be recycled to continue the cellulose surface esterification reaction.

[0042] Example 3

[0043] (1) Add 20 mL of 3.5% cellulose nanocrystal suspension, 0.014 g of 4-dimethylaminopyridine (DMAP), and 5 mL of vinyl acetate into a 100 mL conical flask.

[0044] (2) Stopper the conical flask and stir vigorously at room temperature for 5 h. After the reaction is complete, allow to stand, filter, wash the solid, and dry to obtain surface-esterified cellulose. Infrared spectroscopy was used to determine that the surface of the surface-modified cellulose contained a large number of ester groups (Figure 1).

[0045] (3) After adding 0.01 g of potassium carbonate to the filtered liquid for regeneration, the liquid can be recycled to continue the cellulose surface esterification reaction.

[0046] Example 4

[0047] (1) Add 50 mL of 30% microcrystalline cellulose, 1.5 g of 4-dimethylaminopyridine (DMAP), and 20 mL of vinyl acetate into a 100 mL conical flask.

[0048] (2) Stopper the conical flask and stir vigorously at 60°C for 30 minutes. After the reaction is complete, allow to stand, filter, wash the solid, and dry to obtain surface-esterified cellulose. Infrared spectrometry is used to determine that the surface of the surface-modified cellulose contains a large number of ester groups.

[0049] (3) After adding 0.2 g of sodium hydroxide to the filtered liquid for regeneration, the liquid can be recycled and reused to continue the cellulose surface esterification reaction.

[0050] Example 5

[0051] In a 100 mL conical flask, the filtered solution obtained in step (3) of Example 1 was added, along with 2 g of microcrystalline cellulose, 0.3 g of DMAP, and 0.5 g of sodium carbonate. For calibration, the remaining amount of 4-dimethylaminopyridine (DMAP) was 1.0 g and the remaining amount of vinyl acetate was 19 mL. The conical flask was stoppered and stirred vigorously at room temperature for 1 hour. After the reaction was complete, the mixture was allowed to stand, filtered, and the solid was washed and dried to obtain surface-esterified cellulose. Infrared spectroscopy was used to determine that the surface of the surface-modified cellulose contained a large number of ester groups (Figure 1).

[0052] Example 6

[0053] (1) Add 50 mL of water, 2 g of cotton linter, 1 g of diethanolamine, and 20 mL of vinyl butyrate into a 100 mL conical flask.

[0054] (2) Stopper the conical flask and stir vigorously at 60°C for 30 min. After the reaction is complete, allow to stand, filter, wash the solid, and dry to obtain surface-esterified cellulose. Infrared spectroscopy was used to determine that the surface of the surface-modified cellulose contained a large number of ester groups (Figure 1).

[0055] (3) After adding 0.5 g of sodium carbonate to the filtered liquid for regeneration, the liquid can be recycled to continue the cellulose surface esterification reaction.

[0056] Example 7

[0057] (1) Add 50 mL of water, 2 g of microcrystalline cellulose, 2 g of 4-dimethylaminopyridine (DMAP), and 20 mL of vinyl butyrate to a 100 mL conical flask.

[0058] (2) Stopper the conical flask and stir vigorously at 60°C for 5 minutes. After the reaction is complete, allow to stand, filter, wash the solid, and dry to obtain surface-esterified cellulose. Infrared spectrometry was used to determine that the surface of the surface-modified cellulose contained a large number of ester groups.

[0059] (3) After adding 0.5 g of sodium carbonate to the filtered liquid for regeneration, the liquid can be recycled to continue the cellulose surface esterification reaction.

[0060] In order to further demonstrate the beneficial effects of the present invention and to better understand the present invention, the technological progress of the present invention is determined through the following comparative examples, but they should not be understood as limiting the present invention. The product properties obtained by other measurement experiments conducted by technicians in this field based on the above-mentioned invention content and the applications based on the above-mentioned properties are also considered to fall within the scope of protection of the present invention.

[0061] Comparative Example 1

[0062] (1) Add 50 mL of water, 2 g of microcrystalline cellulose, no organic base catalyst, and 20 mL of vinyl acetate into a 100 mL conical flask.

[0063] (2) Stopper the conical flask and stir vigorously at 60°C for 5 min. After the reaction is complete, allow to stand, filter, wash the solid, and dry. Infrared spectroscopy was used to determine the sample structure, revealing that the cellulose surface was not successfully modified with ester groups (Figure 1).

[0064] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for achieving cellulose surface esterification in an aqueous solution, characterized in that: The following steps are involved: (I) adding a cellulose raw material to an aqueous solution to form a mixed solution, controlling the concentration of the mixed solution to be 0.1% to 30% by mass, and stirring uniformly; (II) adding an organic base catalyst and a vinyl ester esterifying agent to the mixed solution obtained in step (I), stirring vigorously, reacting at a constant temperature for a period of time, and allowing the mixture to stand after the reaction, filtering, and drying to obtain a cellulose surface esterification product; (III) After adding a regeneration agent to the liquid obtained by filtration in step (II), the liquid is recycled for subsequent cellulose surface esterification reactions.

2. The method according to claim 1, characterized in that The cellulose in step (I) includes one or more of pulp, cotton, microcrystalline cellulose, nanocellulose, and wood fiber.

3. The method according to claim 1, characterized in that The organic base catalyst in step (II) comprises at least one of 4-dimethylaminopyridine, monoethanolamine, diethanolamine, triethanolamine, monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, and triethylamine, and the amount of the organic base catalyst added is 1 to 100% of the mass of the cellulose raw material.

4. The method according to claim 1, wherein The vinyl ester esterifying agent in step (II) includes one or more of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl octanoate, vinyl caprate, and vinyl laurate, and the amount of the vinyl ester esterifying agent added is not less than 5% of the mass of the cellulose raw material.

5. The method according to claim 1, wherein The reaction temperature of step (II) is 0°C-70°C, and the reaction time is 1 min-5 h.

6. The method according to claim 1, characterized in that The step (III) comprises replenishing cellulose raw material or a cellulose raw material mixture, a vinyl ester esterifying agent, an organic base catalyst, and a regeneration agent, and obtaining the content of the vinyl ester esterifying agent and the organic base catalyst in the liquid by detection, calibration, and filtration, so that the mass fraction of the cellulose raw material mixture in the reaction solution is 0.1% to 30%, the mass of the organic base catalyst is not less than 1% of the mass of the cellulose raw material, and the mass of the vinyl ester esterifying agent is not less than 5% of the mass of the cellulose raw material.

7. The method according to claim 1 or 6, characterized in that The regeneration reagent includes at least one of sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, calcium hydroxide, and ammonia water, and the added amount of the regeneration reagent is not higher than the initial molar amount of the organic base catalyst.

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