Preparation of nanocellulose by TEMPO oxidation method: principles, processes and applications

As a high-performance bio-based material, nanocellulose shows broad application prospects in the fields of energy, environmental protection, biomedicine and other fields due to its unique nanostructure, excellent mechanical properties and degradability. Among the many preparation methods, the TEMPO oxidation method has become the mainstream technology for the preparation of carboxylated nanocellulose (TOCN) due to its high selectivity, mild reaction conditions and product stability. This article will systematically explain the technical points of preparing nanocellulose by TEMPO oxidation from the aspects of reaction principle, process flow, performance control and industrialization challenges.

1. Reaction principle of TEMPO oxidation method

The core of the TEMPO oxidation method is to use 2,2,6,6-tetramethylpiperidine nitroxide radical (TEMPO) as a catalyst, under the synergistic effect of sodium hypochlorite (NaClO) and sodium bromide (NaBr), to selectively oxidize the primary hydroxyl group (-CH₂OH) at the C6 position of the cellulose molecular chain and convert it into a carboxyl group (-COOH). The reaction process is divided into two steps:

1. Free radical generation : TEMPO generates oxidized state TEMPO⁺ under the oxidation of NaClO and NaBr, and NaClO is reduced to NaCl at the same time.

2. Primary hydroxyl oxidation : TEMPO⁺ oxidizes the primary hydroxyl group at the C6 position of cellulose to an aldehyde group, and then the aldehyde group is further oxidized to a carboxyl group.

The reaction was carried out under alkaline conditions (pH 10-10.5), and the pH of the reaction system was maintained stable by adding NaOH dropwise. The introduction of carboxyl groups significantly enhances the electrostatic repulsion between cellulose molecules, weakens the hydrogen bonding, and loosens the originally tight fiber structure. Subsequent mechanical processing (such as high-pressure homogenization or ultrasonic waves) can peel the fibers into nanoscale single fibers.

2. Process flow of TEMPO oxidation method

1. Raw material pretreatment

Applicable raw materials include natural cellulose such as wood pulp, cotton pulp, bamboo pulp and agricultural waste (such as straw, bagasse). Preprocessing steps include:

· Bleaching : remove lignin and hemicellulose and improve cellulose purity;

· Drying : Dry the pretreated fiber to a constant weight to facilitate subsequent weighing;

· Dispersion : Disperse dry fibers in deionized water to form a uniform suspension.

2. Oxidation reaction

Taking 1 gram of dry cellulose as an example, the typical reaction system composition is as follows: | Components | Dosage | Function | |------------|---------------|--------------------------| | TEMPO | 0.016 g (0.1 mmol) | Catalyst, start free radical reaction | | NaBr | 0.1 g (1 mmol) | Collaborative catalysis, improve reaction efficiency | Maintain pH 10-10.5 | | Deionized water | 100 mL | Solvent medium |

Reaction conditions:

· Temperature : 20-25℃ (mild conditions to avoid cellulose degradation);

· Time : 2-4 hours (extension of time can increase the carboxyl content, but excessive oxidation can lead to chain breakage);

· Stirring speed : 400-600 rpm (to ensure uniform reaction system).

3. Termination of reaction and washing

· Terminate the reaction : Add ethanol or sodium sulfite to quench the remaining oxidant;

· Centrifugal washing : Centrifuge and wash with deionized water 3-5 times to remove salt and small molecule by-products;

· Dialysis purification : Use a dialysis bag with a molecular weight cutoff of 10,000 Da to dialyze for more than 48 hours to further remove residual small molecules.

4. Fiber dissociation and dispersion

Oxidated cellulose easily dissociates into single nanofibers due to negative charge (carboxy) on the surface:

· Stirring and dispersing : 30 minutes of mechanical stirring for initial dispersion;

· Ultrasonic treatment : 20 kHz ultrasonic treatment for 10-15 minutes to further break up fiber aggregation;

· High-pressure homogenization (optional): 600-1000 bar high-pressure homogenization treatment to obtain a more uniform nanofiber dispersion.

3. Product performance and structural characterization

1. Key performance parameters

index	Numerical range	Feature description
Fiber diameter	3-20nm	Nanoscale size, high specific surface area
length	Hundreds of nanometers to several micrometers	Aspect ratio >100 , forming a network structure
Carboxylic content	0.5-1.5 mmol/g	High reactivity, easy for functional modification
Zeta potential ( pH7 )	-30 ~ -50 mV	Strong negative charge, excellent dispersion stability
form	Gel-like, clear or creamy white	Stable suspension in water, no settling for several months

2. Structural characterization methods

· FTIR spectrum : Carboxyl C=O stretching peak at 1730 cm⁻¹, confirming the occurrence of oxidation;

· NMR (¹³C CP/MAS) : The C6 carbon signal shifts left to 175 ppm, indicating carboxylation;

· XRD : Maintain cellulose type I crystal structure, and the crystallinity is not significantly affected;

· TEM/AFM : observe fiber diameter and length, and verify nanoscale dimensions;

· Zeta potentiometer : evaluate dispersion stability and guide process optimization.

4. Process parameter optimization and industrialization challenges

1. Optimization of key parameters

parameter	Enhanced effects	risk
NaClO dosage	Increase the degree of carboxylation	Peroxidation causes chain breakage and reduces mechanical properties
Reaction time	Increase carboxyl density	Affects the stability of the crystallization zone and easily produces by-products
temperature	Accelerate reaction rate	High temperatures can easily cause side reactions and destroy the fiber structure.
NaBr concentration	Improve catalytic efficiency	Insufficient concentration leads to incomplete reaction

2. Industrialization challenges and solutions

· High catalyst cost : adopt TEMPO reuse technology or develop low-cost alternative catalysts (such as TEMPO derivatives);

· Heavy water treatment burden : introduce membrane separation technology (ultrafiltration /nanofiltration) to recover the reaction liquid and reduce waste liquid discharge;

· Easily agglomerated after drying : The product is packaged in the form of slurry or freeze-dried powder to avoid fiber aggregation during the drying process;

· Lack of standardization : Promote international standardization cooperation and establish a unified performance evaluation and quality control system.

5. Application prospects and industry direction

TEMPO oxidized nanocellulose has shown broad potential in multiple high value-added industries due to its excellent dispersion, surface reactivity and biocompatibility:

· Medical hydrogels and dressings : Carboxyl groups can load drugs or growth factors to achieve controlled release;

· Food and packaging materials : degradable, high barrier properties, replacing traditional plastics;

· Flexible electronics and 3D printing : good rheology, suitable as composite printing material;

· Environmental purification : Carboxyl groups absorb heavy metals, dyes and other pollutants;

· New energy : used in lithium battery separators and conductive composite materials to improve battery performance.

Conclusion

As an efficient and green nanocellulose preparation technology, the TEMPO oxidation method can achieve nanoscale dissociation and functional modification of cellulose by precisely controlling the oxidation reaction conditions. With the continuous breakthroughs in low-cost catalysts, green reaction systems and continuous production technology, this technology is expected to achieve larger-scale commercial applications in the future and provide innovative material solutions for sustainable development.