Carbon Molecular Sieve Introduction

1. What is carbon molecular sieve?

Carbon molecular sieves (CMS) are a special class of activated carbons. They are non-crystalline or amorphous and quite unlike zeolite molecular sieves which have a well ordered crystalline structure. However, pore size distribution of these materials is not always strictly discrete and furthermore, molecules are not hard spheres; they can sometimes squeeze into narrow pores. The distinction between activated or porous carbons and carbon molecular sieves is not clearly defined. Carbon molecular sieves have most of the pores in the molecular size range but some conventional activated carbons also have very small pores. The main distinction is that activated carbons separate molecules through differences in their adsorption equilibrium constants. In contrast, an essential feature of the carbon molecular sieves is that they provide molecular separations based on rate of adsorption rather than on the differences in adsorption capacity. This behavior is clearly evident in pressure swing adsorbers (PSA) where gas dynamics dominate. The separation of nitrogen from air by PSA is the single most important application of CMS.

2. Difference between carbon molecular sieve and activated carbon

Activated carbon is a predominantly amorphous solid that has an extraordinarily large internal surface area and pore volume. Its history can be traced to 350 BC when it was used by Egyptian for the reduction of copper, zinc, and tin ores. In modern day, it is still widely used and is produced as a powder, granule, or preformed shapes (pellet, extrudate, and block). The structure of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aliphatic bridging groups. Activated carbon is mostly nongraphitic, remaining amorphous; a randomly cross-linked network inhibits reordering of the structure. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of the carbonization and activation.

Carbon molecular sieve differ from activated carbon mainly in the pore size distribution and surface area. While activated carbons have a broad range of pores, with a typical average pore diameter of 20 A, carbon molecular sieves have a more narrow pore size distribution, with pore sizes in the range of 3 - 5 A. Typical surface areas for a carbon molecular sieve are in the range of 250-400 m2/g, while the micropore volume is about 0.15-0.25 cm3/g.

3. Application of carbon molecular sieve

The primary use of carbon molecular sieves is for the separation and purification of gases. Carbon molecular sieve uses the characteristics of sieving to achieve the purpose of separating oxygen and nitrogen. When the molecular sieve adsorbs impurity gas, the macropores and mesopores only play the role of channels, transporting the adsorbed molecules to the micropores and submicropores, and the micropores and submicropores are the real adsorption volume. As shown in the previous figure, the carbon molecular sieve contains a large number of micropores. These micropores allow molecules with a small dynamic size to rapidly diffuse into the pores while restricting the entry of large-diameter molecules. Due to the difference in the relative diffusion rate of gas molecules of different sizes, the components of the gas mixture can be effectively separated. Therefore, when manufacturing carbon molecular sieves, according to the size of the molecules, the distribution of micropores inside the carbon molecular sieve should be 0.28 to 0.38 nm. Within the size range of the micropores, oxygen can quickly diffuse into the pores through the pores of the micropores, but it is difficult for nitrogen to pass through the pores of the micropores, thereby achieving oxygen and nitrogen separation. The pore size of the carbon molecular sieve is the basis for the separation of oxygen and nitrogen. If the pore size is too large, oxygen and nitrogen molecular sieves can easily enter the pores and cannot separate; and if the pore size is too small, neither oxygen nor nitrogen can enter. In the micropores, there is no separation effect.

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