IMPREGNATED ACTIVATED CARBON MATERIALS FOR RESPIRATORY PURPOSE. CHEMISORPTION OF SULFUR DIOXIDE

Abstract

The review is devoted to the use of impregnated activated carbon materials as chemisorbents of sulfur (IV) oxide. General methods for obtaining ordinary activated carbon, preparation of raw materials, their chemical activation with alkalis and acids followed by heat treatment (carbonization) in an inert environment or in the presence of a gaseous oxidizer, the role of acid-base and redox catalysts in this process are considered. The influence of the chemical composition of the  activated carbon surface, the presence of functional groups, and their acid-base properties, as well as the products of surface reactions on the peculiarities of sulfur (IV) oxide  adsorption is analyzed from the point of view of SO2 removal efficiency and the possibility of SO2 regeneration. An important role in these processes is played by the  pore size, the possibility of co-adsorption of water, and the presence of an oxidant. The nature of adsorbent-adsorbate interactions on the surface of activated carbon, their ener­gy, in particular, the contribution of so-called "physical" adsorption, van der Waals forces, hydrogen bonding, and the influence of surface functional groups are discussed. The activation of carbon raw materials with nitrogen-containing compounds leads to the N-doping of the surface, which increases the efficiency of SO2 adsorption, facilitating not only van der Waals and electrostatic interactions, but also S←N binding. The influence of oxygen and oxygen-containing functional groups on SO2 adsorption is also discussed. To obtain impregnated activated carbon for SO2 absorption, the original activated carbon of the required quality is impregnated with solutions of inorganic and organic compounds that remain on the inner surface of the activated carbon after drying. Impregnation blocks partly  the porosity of activated carbon, but makes it more capable of chemical adsorption. Chemisorption, in which certain chemical bonds are formed between the surface of the activated carbon and the compound being adsorbed, is more selective than physical adsorption, where the size of  molecules is critical for an effective capture process. It can be noted that unlike inorganic alkalis, which spoil the porous structure of activated carbon, treatment with a solution of ammonia or organic N-containing bases promotes SO2 absorption. A special place in gas purification is occupied by activated carbon impregnated with ionic liquids, non-aqueous solvents being used for impregnation. A separate issue of the chemisorption of sulfur (IV) oxide by samples of impregnated activated carbon based on d-metals will be discussed in detail below.