Activated carbons lignocellulosic materials

Phosphoric acid and zinc chloride are activating agents usually used for the activation of lignocellulosic materials which of coal precursors or chars have not been previously carbonized. Contrarily, cilkaline metal compounds, usually KOH, are used for the activation (Table 8.5). 

Chemical activation of lignocellulosic materials is mainly directed toward the preparation of powdered activated carbons (Studies of Caturla etal, 1991 Molina-Sabio etal., 1995, 2003 Almansa et al., 2004). As such, with the precursor being finely divided, the homogeneity of the mixture with the reagent is ensured. However, if the objective is to prepare granular activated carbon the impregnation step has to be carried out with special care to ensure an intimate contact between the precursor and the reagent. 

Activated carbons [171-182] are amorphous materials showing highly developed adsorbent properties. These materials can be produced from approximately all carbon-rich materials, including wood, fruit stones, peat, lignite, anthracite, shells, and other raw materials. The properties of the produced adsorbent materials will depend not merely on the preparation technique but as well on the carbonaceous raw material used for their production. Actually, lignocellulosic materials account for 47% of the total raw materials used for active carbon production [178],... 

Rodriguez-Reinoso F, Molina-Sabio M (1992) Activated carbons from lignocellulosic materials by chemical and/or physical activation an overview. Carbon 30(7) 1111-1118... 

Gonzalez J.C.,.Gonzalez M.T, Molina-Sabio M. Rodriguez-Reinoso F. (1995) Porosity of activated carbons prepared from different lignocellulosic materials. Carbon, 33,1175,... 

The carbonization of lignocellulosic materials with metal chlorides (the basis of chemical activation). 

Physical activation has traditionally included a controlled gasification of the carbonaceous material that has previously been carbonized, although occasionally the activation of the precursor can be done directly. Many different carbonaceous precursors have been employed for physical activation lignocellulosic materials, coals, woods, and materials of polymeric origin. The samples are typically treated to 800-1100°C with an oxidant gas, mainly CO2 or steam, so that carbon atoms are removed selectively. Although this process obviously involves a chemical reaction (and is not merely a physical process), it is known as physical activation. 

To limit the length of the chapter, the discussion of these issues will be restricted to carbon materials (any type, independent of their origin, e.g., coals, chars, nanotubes). Hence, other carbonaceous materials (such as polyaromatic hydrocarbons, resins, polymers, lignocellulosic materials, pitches) that can also be activated when heated in the presence of hydroxides will not be considered. 

Activation results with KOH and NaOH are not necessarily the same. The relative effectiveness depends on the precursor used, especially on its structural order NaOH appears to be better for carbons without structural order (e.g., subbituminous coal, lignite, and lignocellulosic materials), whereas KOH appears to be better for those having some structural order (e.g., anthracite, heat-treated coals, MWCNT). 

Activated carbons, with microporosity and controlled pore-size distributions, are prepared fiom (i) synthetic carbon precursors mainly of polymeric type (polyimide, polyvinyl chloride, resins, etc.) and (ii) the more conventional precursors, such as lignocellulosic materials. 

Lignocellulosic materials constitute the more commonly used precursor and account for around 45 wt% of the total raw materials used for the manufacture of activated carbon. Low contents of inorganic materials are important to produce activated carbons with low ash content, but relatively high volatile content is also needed for the control of the manufacturing... 

Hard coals (high-rank bituminous coals) are the preferred precursor in many countries, because they can be used both for production of inexpensive activated carbons, and also for the more expensive granular, hard carbons with well-developed porous structures. As-received coals have some porosity (which decreases with increasing rank of coal), and consequently further treatments are needed to increase the porosity. Low rank coals (peat, lignite, brown coal), which do not fuse on carbonization, are used to produce activated carbons with a wide pore-size distribution. The yield of activated carbons from coal is generally larger than for lignocellulosic materials, above 30 wt%. 

---