With the continuous progress of science and technology, there are increasingly diverse types of activated carbon fibers. According to their different precursor materials, they can be divided into viscose-based, phenolic-based, polyacrylonitrile-based, asphalt-based, polyvinylidene chloride-based, polyimide-based, PBO-based, polystyrene-based, polyvinyl alcohol-based fibers, etc. The most widely used in industry are the first four types.
Due to different raw materials, the production process varies. By considering various raw materials into production processes of activated carbon fibers, the entire process of preparing activated carbon fibers can be summarized as: raw material fibers, carbonized fibers, activated carbon fibers. There are two types of pretreatment for raw material fibers, each with a different function. One is inorganic salt soaking pretreatment, which can improve the structure, properties and product yield of activated carbon fibers. The other is pre-oxidation stabilization treatment, which can maintain the fiber shape, prevent the melting and deformation during the carbonization process, and form a stable structure. The activation reaction is the main process for generating rich micropores, high specific surface area and functional groups in activated carbon fibers. The commonly used activation methods mainly include physical activation with CO2 or water vapor and chemical activation with ZnCl2, H2PO4, KOH. The treatment temperature is between 700 and 1000℃. Commonly used activators are water vapor and carbon dioxide.
After activation, carbon atoms in carbon fibers mainly exist in the form of graphite microcrystalline layers and stacked layers. The structural characteristics of activated carbon fibers include the developed specific surface area, which can generally reach a specific surface area of 1000 to 1600 m2/g, forming a large number of micropores with a pore size of 10 μm to 40 μm, and the pore distribution is narrow and uniform. The micropore area accounts for about 90% of the total volume. The fibers contain many irregular structures - heterocyclic structures or microstructures containing surface functional groups, which have a high surface energy. This creates a strong molecular field formed by the combined action of micropores and walls, providing a high-pressure system for adsorbed molecules and chemical changes. The diffusion path of the adsorbed substance to the adsorption site is shorter and the driving force is greater due to the concentration of pore size distribution in activated carbon fibers. This is the reason why activated carbon fibers have a larger specific surface area, faster adsorption and desorption rates, and higher adsorption efficiency than activated carbon.
By appropriate surface modification, the type and content of chemical groups on the surface of activated carbon fibers can be changed to meet the requirements of specific substances for efficient adsorption and conversion. Activated carbon fibers are usually suitable for adsorbing low molecular weight molecules in gas phase and liquid phase. When the micropore size of the adsorbent is about twice the critical size of the adsorbate molecule, the adsorbate is more easily adsorbed. The purpose of adjusting the pore size is to make the fine pores of the activated carbon fibers comparable to the size of the adsorbate molecule, and the modification needs to consider the influence of both physical and chemical structures.