Cellulose is a complex polysaccharide composed of many glucose units connected by β-1,4-glycosidic bonds. It is the main component of plant cell walls and gives plant cell walls strong structural support and toughness. Due to the long cellulose molecular chain and high crystallinity, it has strong stability and insolubleness.
(1) Properties of cellulose and difficulty in dissolving
Cellulose has the following properties that make it difficult to dissolve:
High crystallinity: The cellulose molecular chains form a tight lattice structure through hydrogen bonds and van der Waals forces.
High degree of polymerization: The degree of polymerization (i.e. the length of the molecular chain) of cellulose is high, usually ranging from hundreds to thousands of glucose units, which increases the stability of the molecule.
Hydrogen bond network: Hydrogen bonds are widely present between and within cellulose molecular chains, making it difficult to be destroyed and dissolved by general solvents.
(2) Reagents that dissolve cellulose
Currently, the known reagents that can effectively dissolve cellulose mainly include the following categories:
1. Ionic Liquids
Ionic liquids are liquids composed of organic cations and organic or inorganic anions, usually with low volatility, high thermal stability and high adjustability. Some ionic liquids can dissolve cellulose, and the main mechanism is to break the hydrogen bonds between cellulose molecular chains. Common ionic liquids that dissolve cellulose include:
1-Butyl-3-methylimidazolium chloride ([BMIM]Cl): This ionic liquid dissolves cellulose by interacting with hydrogen bonds in cellulose through hydrogen bond acceptors.
1-Ethyl-3-methylimidazolium acetate ([EMIM][Ac]): This ionic liquid can dissolve high concentrations of cellulose under relatively mild conditions.
2. Amine oxidant solution
Amine oxidant solution such as a mixed solution of diethylamine (DEA) and copper chloride is called [Cu(II)-ammonium solution], which is a strong solvent system that can dissolve cellulose. It destroys the crystal structure of cellulose through oxidation and hydrogen bonding, making the cellulose molecular chain softer and more soluble.
3. Lithium chloride-dimethylacetamide (LiCl-DMAc) system
The LiCl-DMAc (lithium chloride-dimethylacetamide) system is one of the classic methods for dissolving cellulose. LiCl can form a competition for hydrogen bonds, thereby destroying the hydrogen bond network between cellulose molecules, while DMAc as a solvent can interact well with the cellulose molecular chain.
4. Hydrochloric acid/zinc chloride solution
The hydrochloric acid/zinc chloride solution is an early discovered reagent that can dissolve cellulose. It can dissolve cellulose by forming a coordination effect between zinc chloride and cellulose molecular chains, and hydrochloric acid destroying the hydrogen bonds between cellulose molecules. However, this solution is highly corrosive to equipment and is limited in practical applications.
5. Fibrinolytic enzymes
Fibrinolytic enzymes (such as cellulases) dissolve cellulose by catalyzing the decomposition of cellulose into smaller oligosaccharides and monosaccharides. This method has a wide range of applications in the fields of biodegradation and biomass conversion, although its dissolution process is not completely chemical dissolution, but is achieved through biocatalysis.
(3) Mechanism of cellulose dissolution
Different reagents have different mechanisms for dissolving cellulose, but in general they can be attributed to two main mechanisms:
Destruction of hydrogen bonds: Destroying the hydrogen bonds between cellulose molecular chains through competitive hydrogen bond formation or ionic interaction, making it soluble.
Molecular chain relaxation: Increasing the softness of cellulose molecular chains and reducing the crystallinity of molecular chains through physical or chemical means, so that they can be dissolved in solvents.
(4) Practical applications of cellulose dissolution
Cellulose dissolution has important applications in many fields:
Preparation of cellulose derivatives: After dissolving cellulose, it can be further chemically modified to prepare cellulose ethers, cellulose esters and other derivatives, which are widely used in food, medicine, coatings and other fields.
Cellulose-based materials: Using dissolved cellulose, cellulose nanofibers, cellulose membranes and other materials can be prepared. These materials have good mechanical properties and biocompatibility.
Biomass energy: By dissolving and degrading cellulose, it can be converted into fermentable sugars for the production of biofuels such as bioethanol, which helps to achieve the development and utilization of renewable energy.
Cellulose dissolution is a complex process involving multiple chemical and physical mechanisms. Ionic liquids, amino oxidant solutions, LiCl-DMAc systems, hydrochloric acid/zinc chloride solutions and cellolytic enzymes are currently known to be effective agents for dissolving cellulose. Each agent has its own unique dissolution mechanism and application field. With the in-depth study of the cellulose dissolution mechanism, it is believed that more efficient and environmentally friendly dissolution methods will be developed, providing more possibilities for the utilization and development of cellulose.
Post time: Jul-09-2024