High-Performance Materials from Bio-based Feedstocks. Группа авторов

[52].

(2.7)

(2.8)

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 agents, the produced activated carbon only contained hydrophobic groups (C–H, C–C, and C=C) [48]. As the hydrocarbon groups in the oil‐rich biomass could be cracked during the carbonization process, it generated more aromaticity of the activated carbon. Therefore, not only the type of chemical activating agent but also the category of biomass feedstock influence the chemical properties of activated carbon.

       2.3.2.2 Physical Activation

For the development of the specific surface area and porous structure of activated carbon, the physical activation process is a favorable approach. Physical activation is achieved by carbonization in the presence of oxidizing gases such as steam, carbon dioxide (CO₂), or a mixture of these. This process resembles a partial gasification process which is generally carried out at temperatures between 700 and 1000 °C at atmospheric pressure. The furnace for the physical activation process is constructed with a gas flow unit, and a horizontal tube furnace or a fluidization system in a vertical tube are frequently selected [55, 56]. The oxygen in the activator gas molecule reacts with the carbon atom of biomass resulting in the generation of carbon monoxide (CO). The precise reaction pathways depend on the type of oxidizing gas and the activation temperature. Physical activation by steam and CO₂ during carbonization occurs through the endothermic reactions as shown in Eqs. (2.9) and (2.10).

(2.9)

(2.10)

      As previously mentioned, the physical activation is similar to partial gasification because the oxygen atoms of the oxidizing gas molecules can react with the carbon atom inside the biochar during activation. From the steam activation in Eq. (2.9), the carbon atom reacts with water molecule yielding CO, which can increase the porosity and surface area of activated carbon. Other prominent points of physical activation are to provide less contamination of activated carbon than chemical activation [57]. Physical activation does not require any neutralization procedure for the removal of the oxidizing agent. So, this process is more environmentally friendly. However, the use of fresh biomass or cellulosic material tends to produce ash during the physical activation process, which results in low activated carbon yields [3].

The activation conditions are one of the important factors considered in promoting a high surface area and high product yield. The proper carbonization period is more influential on the final properties of activated carbon than the activation period. In the work of Rezma et al. [36], the biomass was first carbonized at 1000 °C and the resulting product was activated by CO₂ at 750–900 °C for 30 minutes. The activated carbon presented a very low surface area and product yield [36]. As shown by Grima‐Olmedo et al. [19], the surface area of carbonized eucalyptus was improved when the temperature was raised from 600 to 800 °C. The addition of CO₂ during carbonization strongly enhanced the surface area of the produced activated carbon [19].

      Both micro‐ and mesopores can be generated through physical activation. For example, the micropores inside carbonized rubber wood sawdust could be changed into mesopores via steam activation, resulting in a high surface area of 1134 m² g⁻¹ [58]. The surface functional groups of physically activated carbons correspond to those produced by chemical activation [39]. The factors that affect the physical and chemical properties of activated carbon can be prioritized as: (i) carbonization and activation temperatures, (ii) type of feedstock, (iii) particle size of feedstock, (iv) heating rate, (v) gas flow rate, and (vi) activation time [28]. This information is significant for the design of the physical activation process.

      2.3.3 Hydrothermal Carbonization

      Hydrothermal carbonization (HTC) is an environmentally friendly thermochemical process for biomass conversion. This process operates at temperatures of 180–300 °C under the pressure of 8–20 bar in water [59]. The thermal processing at high pressure in water destructs the biomass structure and the resulting carbon material is called a hydrochar. The HTC is proposed as a productive process for fine biomass particles because it presents a higher yield of the bio‐based carbon product compared to regular carbonization [60]. Moreover, carbonization under high vapor pressure of water can produce hydrochar with uniform particles.

According to the hydrothermal reaction of the lignocellulosic material, the cellulose and hemicellulose are hydrolyzed to obtain CO₂ as a by‐product according to Eq. (2.11).

(2.11)