The Preparation Processes of Nanocellulose: From Laboratory to Industrialization — Exploring the Frontiers of Nanocellulose Science

Acid Hydrolysis Preparation
Acid hydrolysis of cellulose primarily refers to dilute acid hydrolysis. The dilute acid mainly acts on the amorphous regions; it does not dissolve cellulose but facilitates hydrolysis in a two-phase system. After dissolving the amorphous regions, the crystalline regions remain, resulting in nanocrystalline cellulose with high crystallinity and a well-defined crystalline structure.
Common inorganic acids used in acid hydrolysis for NCC preparation include sulfuric acid, hydrochloric acid, and phosphoric acid, with sulfuric acid being the most frequently used. Hydrochloric acid hydrolysis yields NCC with minimal surface charge, while sulfuric acid hydrolysis produces highly stable aqueous suspensions. However, this method generates large amounts of waste acid and impurities, requires high reaction equipment standards, and faces challenges in post-reaction recovery. Despite these drawbacks, the process is well-established and has been industrialized.

Enzymatic Hydrolysis Preparation
Enzymatic hydrolysis involves the use of cellulase to selectively break down amorphous cellulose, leaving behind crystalline cellulose. Commonly used raw materials include lignocellulose, various bacterial celluloses, and microcrystalline cellulose (MCC). NCC produced this way can serve as a reinforcing agent in composites to enhance mechanical properties.
Enzymatic hydrolysis operates under mild conditions, offers high specificity, and utilizes renewable resources (enzymes and cellulase), making it significant for sustainable development. It is anticipated that enzymatic methods will become a major research focus in the future.

Mechanical Preparation
Mechanical methods rely on external forces—such as high-shear mixing, grinding, microfluidization, high-pressure homogenization, and ultrasonication—to break down the cell walls of higher plants, releasing nanoscale cellulose fibers. Alternatively, natural fiber bundles can be directly fragmented into nanoscale cellulose fibers.
Mechanical preparation of NCC requires no chemical reagents, minimizing environmental impact. However, the resulting NCC tends to have a broad particle size distribution. Additionally, the process demands specialized equipment and consumes high energy. High-pressure homogenization and chemi-mechanical methods are both categorized under mechanical preparation.

Hot-Pressing Preparation
The hot-pressing method does not require synthetic polymers or the separation of hemicellulose and lignin. It can produce microfibrillated cellulose with ultra-strong inter-fiber bonding strength.

Biological Preparation
Cellulose produced via microbial synthesis is often referred to as bacterial cellulose. Compared to natural plant cellulose, bacterial cellulose features an ultra-fine network fiber structure, with each filament composed of nanoscale microfibrils.
Its biosynthesis process is controllable, facilitating industrialization and commercialization. However, bacterial cellulose production is complex, time-consuming, costly, and has low yield, making it expensive.

Solvent-Based Preparation
It is feasible to prepare cellulose nanoparticles by adding a non-solvent to a continuously stirred fiber solution. Studies show that faster addition of the non-solver results in smaller NCC particles.

Ionic Liquid Dissolution Preparation
Partial dissolution of microcrystalline cellulose in ionic liquids, followed by casting into thin-film nanocomposites, yields films that are homogeneous, transparent, and highly crystalline.

Conclusion
Nanocellulose is an emerging specialty material with excellent functionalities. Novel nano fine chemicals and nanomaterials represent the frontier and focus of cellulose science. In China, nanocellulose is poised to grow into a major industry.
Research and development of new preparation methods for NCC that are simple, green, low-energy, rapid, and efficient have become a key focus for researchers worldwide.