Preparation method of carboxylated modified cellulose nanocrystals

The preparation of carboxylated modified cellulose nanocrystals (CNCs) are usually improved by introducing carboxylic groups (-COOH). Common preparation methods include chemical oxidation and mechanical treatment. The following are several main methods:

1. TEMPO oxidation method

• Principle : Using TEMPO (2,2,6,6-tetramethylpiperidin-1-oxygen radical) as a catalyst, the hydroxyl group on the surface of cellulose is used in the presence of sodium hypochlorite (NaClO) and potassium hypochlorite (NaBr) Selective oxidation to carboxyl groups.

• step :

1. Suspend the cellulose in water and add TEMPO, NaClO and NaBr.

2. Perform the oxidation reaction by controlling the pH value (usually between 10-11) and the temperature.

3. After the reaction is completed, the residual reagent is removed by dialysis or other means to obtain carboxylated cellulose nanocrystals.。

2. Chloroacetic acid method

• Principle : Use chloroacetic acid (ClCH ₂ COOH) to react with cellulose to convert the hydroxyl group (-OH) on the surface of the cellulose into carboxy group (-COOH).

• step :

1. Dissolve or suspend cellulose in water or organic solvent.

2. Add chloroacetic acid and a basic catalyst (such as NaOH) and carry out the reaction at an appropriate temperature.

3. After the reaction, the product removes unreacted reagents and by-products by washing and filtration to obtain carboxylated cellulose nanocrystals.

3. Citric acid method

• Principle : Use citric acid as a carboxylation to achieve carboxylation by reacting with hydroxyl groups of cellulose.

• step :

1. Mix cellulose with citric acid and react normally in aqueous solution or organic solvent.

2. Reaction is carried out under appropriate temperature and acid-base conditions to form a carboxylation product.

3. After the reaction is completed, carboxylated cellulose nanocrystals are obtained by washing and drying.

4. High-energy mechanical treatment method

• Principle : Through high-energy mechanical methods such as ultrasonic wave, ball milling or high-pressure homogenization, physical changes are made to the surface of cellulose nanocrystals, and carboxyl groups can be introduced.

• step :

1. Suspend cellulose nanocrystals in water.

2. Treat it through ultrasonic waves, ball milling or high-pressure homogenization to modify its surface.

3. After treatment, the carboxy group is further introduced by adjusting the pH value or adding chemical reagents, and finally the modified nanocrystals are washed.

5. Esterification reaction

• Principle : Esterification reaction of cellulose with a carboxylated compound (such as maleic anhydride) to obtain carboxylated cellulose nanocrystals.

• step :

1. Mix cellulose with reagents such as maleic anhydride, and usually react in an organic solvent.

2. During the reaction, cellulose and anhydride compounds are esterified to form a carboxylated product.

3. After the reaction is completed, carboxylated cellulose nanocrystals are obtained by washing and drying.

In practical applications, different modification levels and methods can be selected according to needs to meet specific performance requirements.