2.3.2 Activation
The activation process is an oxidation reaction at elevated temperatures. The objective of the activation process is to enhance the surface area, pore size, and pore volume of the bio‐based carbon material during carbonization, as the highly porous structure of carbon material is advantageous in several applications such as adsorption and catalysis. In order to clarify the distinction between carbonization and activation, the latter process requires both an activating agent and heat. Activation is performed under an oxygen‐free or oxygen‐poor atmosphere depending on the type of activating agent. The product obtained from the activation process is a so‐called activated carbon, which is a highly porous carbon‐rich material. Lignocellulosic materials are promising raw materials for the production of activated carbons since they can provide a great yield of activated carbon together with high porosity and hardness compared to non‐lignocellulosic materials. Previous research showed that activated carbon prepared from lignocellulosic biomass had a surface area in the range of 300–2700 m2 g−1 and various pore sizes [21, 45]. Conventionally, there are two routes in the activation process. First, biomass is carbonized to reduce moisture, volatile matter, and contaminants, and the obtained biochar is subsequently activated with an oxidizing agent and carbonized again. This route can be called a two‐step activation process [46]. A different route is where the biomass is directly activated during the carbonization step, which is a one‐step activation process [28, 39]. The activation method is typically classified into two types depending upon the kind of activating agent employed, and these are chemical and physical activation.
2.3.2.1 Chemical Activation
The utilization of agricultural material as a raw material for the production of activated carbon is usually carried out by chemical activation. The proper temperature for chemical activation should be 400–600 °C. As the chemical activating agent should penetrate and erode the cellulose and lignin and destroy their structures during carbonization, the property of the activating agent should be strongly corrosive. Not only strong acidic and basic agents but also other strong oxidizing agents have been exploited. The most widely used chemical activating agents are potassium hydroxide (KOH), phosphoric acid (H3PO4), and zinc chloride (ZnCl2) [45, 47], but chemical agents such as sodium chloride (NaCl), sodium hydroxide (NaOH), hydrogen peroxide (H2O2), and potassium carbonate (K2CO3) have also been used [48–50]. The chemical activating agents in an aqueous form are typically applied onto the raw materials via impregnation for a desired period of time. The duration of the chemical activation is often around 60–120 minutes, and the ratio of chemical activating agent and raw material is usually between 1 : 1 and 5 : 1. The reaction pathways when using KOH as an activating agent are presented in Eqs. (2.3)–(2.6) as follows [26]:
(2.5)
The activated carbon produced from soybean pods activated by KOH showed the highest surface area of 2245 m2 g−1 [47]. An interesting shortcut in the reaction pathways was taken by employing K2CO3 as an activating agent instead of KOH [48]. The K2CO3 is an initial substance in Eq. (2.4), thus Eq. (2.3) could be eliminated. The activated carbon produced by using K2CO3 also showed an extremely high surface area of 2613 m2 g−1 along with a high pore volume of 1.66 cm3 g−1. However, the KOH showed a higher activation potential than the K2CO3 at low carbonization temperatures with regard to the porosity and surface area of the resulting activated carbon [51], which was ascribed to the difference in melting point. The K2CO3 has a higher melting temperature of 890 °C while the melting point of KOH is 360 °C. When using NaOH as an activating agent, the reaction pathways are presented in Eqs. (2.7) and (2.8) [52].
The reaction steps in heterogeneous catalysis will be discussed in detail in Section 2.4. Since most of the reactant adsorption onto the catalyst surface takes place within the micropores of activated carbon, the number and the dimension of micropores should be carefully considered. Macroporous and mesoporous structures of activated carbon provide the passageway of the reactant to the micropores, hence they impact the overall activity of a heterogeneous catalyst as well. Activated carbon obtained from the chemical activation process regularly presents a microporous or sub‐mesoporous structure with pore diameters in the range of 1–3 nm. Pretreatment of the raw material before activation showed a surprising result. The palm shell was soaked in 5 H2SO4 for 24 hours and the excess H2SO4 was removed by washing with deionized water. Finally, the treated palm shell was activated with H3PO4 at 650 °C for two hours. The pore diameter of the obtained activated carbon was around 20 nm, which corresponds to a mesoporous structure. Furthermore, the proportion of each pore size can be controlled by the combination of several activating agents in one process [50]. Activation with NaCl resulted in activated carbon with a macroporous structure while NaOH provided micropores. The combination of NaCl and NaOH in the appropriate ratio thus enables control of the pore diameter.
Apart from the pore size and structure, also the chemical properties of activated carbon can be modified by chemical activation. The functional groups of activated carbon, especially oxygen‐containing groups, play an important role in catalysis. A variety of chemical activating agents affect the chemical properties of activated carbon. For example, NaOH activation of durian shell produced an activated carbon that contained a large amount of oxygen‐containing groups such as OH, C=O (ketone, aldehyde, lactones, and carboxyl), and C–O (anhydrides) [53]. In contrast, the oxygen‐containing functional groups disappeared from the surface of the activated carbon produced from Euphorbia rigida, which is an oil‐rich biomass [54]. Although ZnCl2, K2CO3, NaOH, and H3PO4 were used as activating
