Nanocellulose preparation method: In-depth analysis of TEMPO oxidation method

1. Introduction

Nanocellulose is a type of nano-scale material obtained by physical, chemical or enzymatic treatment of natural cellulose . It has high strength, high specific surface area, biocompatibility and renewability. It is widely used in cutting-edge fields such as electronic materials, biomedicine, and environmental governance.

Among all preparation methods, the selective oxidation method catalyzed by TEMPO ( 2,2,6,6- tetramethylpiperidin -1- oxy) radicals has become a research hotspot because of its mild oxidation conditions, strong structural controllability and good product stability. This method can selectively oxidize the C6- position primary alcohol hydroxyl group to be a carboxy group to achieve nano-scale fiber dissociation, which is the key route for the preparation of ** TEMPO oxidized nano-cellulose ( TOCNs ) .

2. Detailed explanation of TEMPO oxidation mechanism

TEMPO oxidation reaction is a mild free radical oxidation process , which is essentially the conversion of primary hydroxyl groups to carboxyl groups . The process is as follows:

Catalyst activation : TEMPO generates oxidative state TEMPO⁺ under NaClO oxidation；

Hydroxyselective oxidation : The primary alcohol at C6 position is selectively oxidized to an aldehyde by TEMPO⁺ ;

The aldehyde is further oxidized : the aldehyde is further oxidized into a carboxylic acid in the presence of NaClO .

Key reactions:

Copy Edit

Cellulose–CH2OH + NaClO + TEMPO + NaBr → Cellulose–COOH + NaCl + H2O

This reaction does not damage the cellulose main chain structure and does not cause macromolecules to degrade, so it can maximize the mechanical properties.

3. TEMPO oxidation preparation process

flow chart:

Raw material preparation (slurry dispersion)

TEMPO/NaBr/NaClO joins

Control temperature and stirring, control pH 10.0–10.5

Oxidation reaction for 1–4 hours

Reaction termination (ethanol / sodium sulfite /H₂O₂ )

Wash and remove salt

Ultrasonic or high-pressure homogeneous dissociation

Obtain stable nanocellulose dispersion

4. Key process parameters summary table

Parameter name	Recommended range	illustrate
Raw material concentration	0.5–2 wt%	Avoid excessive thickening to reduce reaction efficiency
TEMPO concentration	0.1–0.5 mmol/g Cellulose	Control the degree of oxidation, excessive amounts will cause side reactions
NaBr concentration	1–5 mmol/g	Co-catalysis
NaClO addition amount	5–15 mmol/g	Determine the final carboxylic content
Reaction pH	10.0–10.5	Maintain alkalinity to promote oxidation
temperature	20–30°C	Gentle conditions to prevent cellulose degradation
Reaction time	1–4 hours	Positively correlated with the degree of oxidation
Dissociation method	Ultrasonic /High-voltage homogeneity	Key steps in nanoification

5. Comparison table of performance before and after TEMPO oxidation

index	Original cellulose	TEMPO Oxidated Nanocellulose (TOCN)
Fiber diameter	10–50 μm	3–10 nm
Suspension appearance	Turbidity settles	Transparent and stable
Carboxylic content	~0 mmol/g	0.5–1.7 mmol/g
Zeta potential	~0 mV	-30 ~ -50 mV
Specific surface area	<10 m²/g	>100 m²/g
Thermal Stability (Tmax)	~340°C	~300°C (slightly reduced)
Dispersibility	差	High, stable in water
Reactive activity	弱	Strong carboxylic functionalization reaction
Application areas	papermaking	Colloids, medical, composite materials

6. Key factors affecting product structure

factor	effect	Recommended optimization direction
The pH is too low	TEMPO inactivation	Maintain an alkaline environment
The reaction time is too long	Main chain break	Monitoring reaction endpoint
Overdose of NaClO	Increased side effects	Strict proportional control
Low purity of raw materials	Introduce interference components	Pretreatment of bleaching or purification is recommended
Inadequate washing	Salt residue	Multiple dialysis or membrane filtration

7. Industrialization challenges and solutions

question	Performance	Response measures
Environmental burden of chlorine-based oxidants	NaClO uses a lot of	Try green oxidation systems, such as H₂O₂-enzyme
High water and energy consumption	Many washing times	Introducing closed-loop water treatment and membrane concentration system
Nanodissociation requires high energy	High cost of ultrasonic /high voltage	Develop low-energy fluid shearing equipment
Unstable product quality	Carboxylic content fluctuations	Introducing online monitoring and automatic dropping system
Lack of commercial standards	No unified performance evaluation	Promote the construction of international standardization cooperation and certification system

8. Post-treatment and functionalization of TEMPO oxidized nanocellulose

TEMPO oxidation is used to introduce cellulose into surface carboxylic groups, making it have the following advantages:

Chemical functionalization platform : –COOH can react with amines, alcohols, and esters to perform drug-loaded and conductive polymer grafting;

Strong metal ions chelating ability : used in wastewater treatment and catalytic support;

Excellent bioadaptive : good carboxyl stability, can be used in tissue engineering scaffolds;

Combination with graphene / nanooxide : improve capacitance performance and electromagnetic shielding ability.

9. Conclusion

Due to its high selectivity, mild reaction conditions and excellent nanofiber dissociation efficiency, TEMPO oxidation has become one of the most representative nanocellulose preparation routes. In the future, with the advancement of green oxidation substitutes, efficient dissociation equipment and intelligent continuous processes, this technology will have more hope to move towards large-scale, low-cost, and functional industrial applications.