Carbon molecular sieve (CMS) is a kind of porous material with uniform and narrow pore size distribution, which has excellent separation and adsorption performance. The detailed introduction is as below:
Definition and Composition
Basic Definition: Carbon molecular sieves are porous carbon-based materials mainly composed of carbon atoms. They are formed by carbonization and activation processes, resulting in a highly porous structure with a large specific surface area.
Chemical Composition: Primarily composed of carbon, they may also contain small amounts of other elements such as hydrogen, oxygen, nitrogen, etc., depending on the raw materials and preparation methods. The carbon atoms are arranged in a disordered and porous manner, forming a complex network of micropores and mesopores.
Structure Characteristics
Pore Structure: Carbon molecular sieves have a hierarchical pore structure, mainly including micropores with a pore size generally in the range of 0.3 – 1.0 nm and mesopores with a slightly larger pore size. This pore structure provides a large specific surface area, usually ranging from 500 to 1500 m²/g, which is crucial for their adsorption and separation capabilities.
Surface Properties: The surface of carbon molecular sieves is rich in various functional groups, such as hydroxyl groups, carboxyl groups, and carbonyl groups. These functional groups can interact with different gas molecules or substances, affecting the adsorption and separation performance.
Preparation Methods
Raw Material Selection: Common raw materials include coal, petroleum pitch, phenolic resins, and biomass materials. Different raw materials have an important influence on the final properties of carbon molecular sieves.
Carbonization: The raw materials are heated in an inert atmosphere at high temperatures (usually 600 – 1000°C) for carbonization to remove volatile components and form a carbonaceous skeleton.
Activation: The carbonized product is further activated by physical or chemical methods. Physical activation uses gases such as steam, carbon dioxide, or air to react with the carbon material at high temperatures to create more pores. Chemical activation involves impregnating the raw material with chemicals such as potassium hydroxide, zinc chloride, or phosphoric acid before carbonization to enhance the pore formation.
Applications
Gas Separation: Widely used in the separation of nitrogen and oxygen in air. They can adsorb oxygen molecules more selectively, enabling the production of high-purity nitrogen through pressure swing adsorption technology. They are also used in the separation of other gas mixtures, such as the separation of carbon dioxide and methane in biogas purification.
Hydrocarbon Separation: In the petrochemical industry, carbon molecular sieves are used to separate different hydrocarbon components, such as the separation of olefins and paraffins, which is of great significance for improving the efficiency of oil refining and chemical production processes.
Water Treatment: They can adsorb organic pollutants and heavy metal ions in water, playing a role in water purification and treatment. For example, they can remove organic dyes, phenols, and heavy metals such as mercury and lead from water.
Advantages
High Selectivity: They have high selectivity for different gas molecules or substances, enabling efficient separation of specific components from complex mixtures.
Good Thermal Stability: They can maintain stable performance in a wide temperature range and can work stably at high temperatures without significant changes in their structure and properties.
Long Service Life: With proper operation and maintenance, carbon molecular sieves have a relatively long service life and can be regenerated and reused multiple times, reducing costs.
Disadvantages
Sensitivity to Impurities: Carbon molecular sieves are easily affected by impurities in the gas or liquid, such as sulfur dioxide, water vapor, and dust, which may reduce their adsorption and separation performance.
High Cost: The preparation process of carbon molecular sieves is relatively complex, requiring high-temperature carbonization and activation steps, resulting in relatively high production costs.
