Nanocellulose acid hydrolysis preparation process: from natural cellulose to cellulose nanocrystals

Cellulose nanocrystals (CNCs) are one of the important types of nanocellulose, with high crystallinity, high mechanical strength and unique nanoscale characteristics. Acid hydrolysis is a classic method for preparing CNC. It hydrolyzes natural cellulose by strong acids, destroys its amorphous region and retains the crystalline region, thereby obtaining nanoscale cellulose crystals. This article will introduce in detail the preparation process and key steps of nanocellulose acid hydrolysis, and provide theoretical support through digital parameter tables.

Principles of Preparation of Cellulose NanoCrystals by Acid Hydrolysis

The core of the acid hydrolysis method is to use strong acids (such as sulfuric acid, hydrochloric acid) to treat natural cellulose. Natural cellulose consists of alternating crystalline regions and amorphous regions. Acid hydrolysis will selectively destroy the amorphous regions, and the crystalline regions are retained due to their dense structure and strong acid resistance. Finally, the cellulose is dissociated into nanoscale crystals to form cellulose nanocrystals (CNCs).

Process flow for preparing CNC by acid hydrolysis

1. Raw material pretreatment

Raw material selection: Commonly used natural cellulose raw materials include wood, cotton, straw, hemp, etc.

Crushing and washing: Crush the raw materials into fine particles and wash them with deionized water to remove impurities and soluble ingredients.

Dry: The washed cellulose is dried at 60-80°C to a constant weight.

2. Acid hydrolysis reaction

Acid solution preparation: Concentrated sulfuric acid (concentration 60-65%) or hydrochloric acid is usually used as the hydrolyzer.

Reaction conditions:

Mass ratio of acid to cellulose: 10:1 to 20:1.

Reaction temperature: 45-60°C.

Reaction time: 30-120 minutes.

Stirring speed: 200-500 rpm to ensure uniform reaction.

Reaction process: The pretreated cellulose is added to the acid solution and the reaction is stirred under constant temperature. Acid hydrolysis destroys the amorphous region of cellulose and releases nanocrystals.

3. Reaction termination and neutralization

Dilution: dilute the reaction solution with a large amount of deionized water to terminate the hydrolysis reaction.

Centrifugation: The CNC suspension was isolated by centrifugation (10,000-15,000 rpm, 10-20 minutes).

Dialysis purification: The CNC suspension was charged into a dialysis bag, dialyzed with deionized water until the pH was close to neutral, and the residual acid and salt were removed.

4. Dispersion and drying

Ultrasonic dispersion: The purified CNC suspension is sonicated (power 200-500 W, time 10-30 minutes) to ensure uniform dispersion of nanocrystals.

dry:

Freeze-drying: After freezing the CNC suspension, the moisture is sublimated under vacuum to obtain fluffy CNC powder.

Spray drying: The CNC suspension is passed through a spray dryer and quickly dried to obtain CNC powder.

Key parameters for preparing CNC by acid hydrolysis

The following are the key parameters and typical ranges for the preparation of CNC by acid hydrolysis:

parameter	Typical range	Influence
Acid concentration	60-65% (sulfuric acid)	Too high concentrations will lead to excessive degradation of cellulose, and too low concentrations will cause incomplete hydrolysis.
Reaction temperature	45-60°C	Too high temperature will accelerate cellulose degradation, and too low temperature will cause slow reaction rate.
Reaction time	30-120 minutes	If the time is too short, the hydrolysis is not complete, and the time is too long, it will lead to the destruction of the crystallization zone.
Acid to cellulose mass ratio	10:1 to 20:1	Too high proportion will increase acid consumption, and too low proportion will cause low hydrolysis efficiency.
Stirring speed	200-500 rpm	Ensure uniform reactions and avoid local overheating or incomplete reactions.
Centrifugal speed	10,000-15,000 rpm	If the speed is too low and the separation is not thorough, the speed is too high, it may damage the CNC structure.
Ultrasonic power	200-500 W	If the power is too low and the dispersion is uneven, the power is too high, it may destroy the CNC structure.
Ultrasound time	10-30 minutes	If the time is too short and the dispersion is insufficient, the time is too long, it may lead to CNC degradation.

Typical product properties of CNC preparation by acid hydrolysis

The following are typical performance parameters of CNC prepared by acid hydrolysis:

Performance metrics	Typical range	illustrate
length	100-500 nm	Nanoscale properties of CNC.
diameter	5-20 nm	Nanoscale properties of CNC.
Crystallization degree	70-85%	High crystallinity is an important feature of CNC.
Tensile strength	200-300 MPa	CNC has excellent mechanical properties.
Young's modulus	10-20 GPa	High rigidity properties of CNC.
Specific surface area	100-300 m²/g	High specific surface area is conducive to the functional application of CNC.
Zeta potential	-30 to -50 mV	The negatively charged surface is conducive to the dispersion and stability of CNC.

Advantages and disadvantages of acid hydrolysis

advantage:

Simple process: low equipment requirements and easy to operate.

Excellent product performance: CNC has high crystallinity, high mechanical strength and nanoscale characteristics.

A wide range of raw materials: a variety of natural cellulose raw materials can be used.

shortcoming:

Environmental Pollution: The use and treatment of strong acids may cause pollution to the environment.

Higher energy consumption: the centrifugation, dialysis and drying steps consume more energy.

Low yield: Some cellulose degrades during acid hydrolysis, and the yield is relatively low.

Optimization and improvement of acid hydrolysis method

In order to improve the efficiency and environmental protection of acid hydrolysis, researchers have proposed a variety of optimization methods:

Green solvent replacement: Use environmentally friendly solvents such as ionic liquids and deep eutectic solvents to replace strong acids.

Enzyme-assisted hydrolysis: Pretreat natural cellulose with cellulase to reduce the amount of acid.

Continuous production: Develop a continuous acid hydrolysis process to improve production efficiency.

Waste acid recycling: Recycling and reuse waste acid through neutralization, crystallization and other methods to reduce environmental pollution.

Application Prospects of Preparation of CNC with Acid Hydrolysis

Due to its excellent performance, CNC has a wide range of application prospects in the following fields:

Composite materials: As a reinforced phase, it improves the mechanical properties of composite materials.

Flexible electronics: used for flexible display screens, sensors and other electronic devices.

Biomedicine: as a drug carrier, tissue engineering stent, etc.

Food industry: As a functional additive such as stabilizers, thickeners, etc.

Acid hydrolysis is a classic method for preparing cellulose nanocrystals, and has the advantages of simple process and excellent product performance. Despite the problems of high environmental pollution and energy consumption, acid hydrolysis still has broad development prospects through process optimization and technological innovation. With the advancement of green chemistry and nanotechnology, acid hydrolysis will provide reliable technical support for the large-scale application of nanocellulose, promoting its wide application in multiple fields.

References

Habibi, Y., et al. (2010). Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chemical Reviews, 110(6), 3479-3500.

Klemm, D., et al. (2011). Nanocelluloses: A New Family of Nature-Based Materials. Angewandte Chemie International Edition, 50(24), 5438-5466.

Moon, RJ, et al. (2011). Cellulose Nanomaterials Review: Structure, Properties and Nanocomposites. Chemical Society Reviews, 40(7), 3941-3994.