The pore size of carbon molecular sieves typically ranges from 0.3 to 0.5 nm, placing them in the category of small-pore molecular sieves.
1. Pore Size Range
Carbon molecular sieves exhibit a highly concentrated pore size distribution, primarily within the 0.3 nm to 0.5 nm range. This precise pore size is the basis for their ability to perform kinetic separation (such as separating nitrogen and oxygen from air).
2. Core Characteristics and Classification
Molecular sieves are generally classified into three categories based on pore size:
• Small-pore molecular sieves (0.3–0.5 nm): Carbon molecular sieves fall into this category. For example, 3A molecular sieves have a pore size of approximately 0.3 nm, and 4A molecular sieves have a pore size of approximately 0.4 nm.
• Medium-pore molecular sieves (0.5–0.8 nm): Such as 5A molecular sieves, which have a pore size of approximately 0.5 nm and exhibit strong nitrogen adsorption capabilities.
• Large-pore molecular sieves (>0.8 nm): Such as 13X molecular sieves, which have a pore size of 0.9–1.0 nm and can adsorb larger molecules.
3. Application Principle
Carbon molecular sieves achieve separation based on minute differences in kinetic diameters. The kinetic diameter of oxygen molecules (0.346 nm) is slightly smaller than that of nitrogen molecules (0.364 nm); consequently, oxygen diffuses more rapidly into the micropores of the carbon molecular sieve and is adsorbed, thereby enriching the nitrogen. The purity of nitrogen produced via this air separation process can typically exceed 99.5%.