in 2004 and presents astonishing properties. Its appearance is like a soft film, and it has a high Young’s modulus and excellent thermal and electrical properties. Previous research reveals that graphene has a very high surface area of over 2000 m2 g−1 and can be easily chemically modified [67]. Within the past decade, numerous investigations have used graphene in various applications along with exploring the practical synthesis procedures of graphene from biomass [68, 69]. Graphene can be synthesized through several methods including micromechanical exfoliation of graphite, chemical vapor deposition (CVD) of graphite, epitaxial graphene growth on silicon carbide, chemical graphitization of graphene oxide, and carbonization of biomass and waste materials [70–74]. Micromechanical exfoliation and CVD of graphite produced good‐quality graphene. The transformation of biomass into graphene can be accomplished via the carbonization of biomass under specific conditions. For example, pyrolysis of camphor leaves at an ultra‐high temperature of 1200 °C under nitrogen or argon atmosphere produced a biochar, which was subsequently reacted with D‐tyrosine as a bio‐based dispersion agent under sonication to obtain graphene [75]. The graphene derived from biomass contained many impurities, whereas the graphene synthesized from graphite is usually purer. To avoid contamination, some researchers used pure lignin to produce fine graphene through a stepwise pyrolysis process [76]. The biomass type may affect the properties of graphene as well. Populus wood was mixed with KOH before carbonization under nitrogen. Surprisingly, the graphene‐like carbon material was found on the structure of the obtained biochar [20]. Sucrose, xylitol, and glucose were also used as starting materials for graphene synthesis. The carbonization of sugar mixed with FeCl3 generated graphene‐like carbon materials, which presented excellent catalytic activity in the hydrogenation of nitrobenzene [77]. Even though graphene is one of the beneficial carbon materials, the production of graphene from biomass without any impurities remains a challenge.
To meet the requirement to be a heterogeneous catalyst or support, the physicochemical properties and thermal stability of the material are crucial as these dictate the catalytic efficiency. As previously described, bio‐based carbon materials have predominant characteristics that benefit their application as a direct catalyst or catalyst support. Biomass‐derived porous carbon materials with high porosity provide a greater number of active sites. Moreover, the pore size and specific surface area can be tailored via the process parameters (temperature, time, heating rate, type of biomass, activating agent, etc.). Several forms of bio‐based carbon materials including granules, pellets, tubes, and sheets can be produced. A variety of natural oxygenated surface functional groups in bio‐based carbon materials is useful for metal adsorption when used as a support. These functional groups can also enhance reactant adsorption in heterogeneous catalysis. Some bio‐based carbon materials are cheaper than commercial catalysts and supports, e.g. zeolite, silica, and alumina. Before going into the details on the applications of promising bio‐based carbon materials in a field of catalysis, some of their properties are summarized in Table 2.3.
Table 2.3 Comparison of typical physical and chemical properties of bio‐based carbon materials [30, 39, 43, 48].
| Bio‐based carbon materials | Surface area (m2 g−1) | Pore volume (cm3 g−1) | Functional groups |
|---|---|---|---|
| Biochar | 100–500 | 0.03–0.25 | –COOH, –OH, C–H, C=C, C=O |
| Hydrochar | 1–43 | 0.007–0.2 | –OH, C=C, C=O, C–C, –CH2, –CH3 |
| Activated carbon (chemical treatment) | 400–2700 | 0.3–1.53 | –OH, C–H, C=C, C–O |
| Activated carbon (physical treatment) | 300–1000 | 0.1–0.99 | –OH, C=O, C–O |
| Graphene | 800–1800 | 0.3–0.9 | C–H, C–OH, C=C |
| Sugar‐derived carbon | 1–5 | — | — |
| Carbon nanotube | 300–600 | 2.5 | –COOH, –OH |
2.4 Fundamentals of Heterogeneous Catalysis
Over 90% of chemical industries (e.g. petroleum, renewable energy, fine chemicals, polymers, and food and pharmaceutical industries) are associated with catalytic processes. In 2019, approximately 72% of industrial catalytic processes used heterogeneous (solid) catalysts [1]. In order to develop more industrially friendly catalysts, many investigations have advanced the use of different bio‐based carbon materials for a wide range of catalytic applications. The studied bio‐based carbon catalysts and supports initially started with biochar, activated carbon, carbon nanotubes (CNTs), mesoporous carbons, and sugar catalysts, but have now been extended to include graphene and its derivatives. The crucial target for these developments is the upgrading of bio‐based carbon materials into direct catalysts or as catalyst supports to maximize catalytic efficiency. A simple process for the preparation of bio‐based carbon materials as well as achieving high conversion and highly desired product yield under mild reaction conditions are aimed for.
An understanding of the mechanisms of heterogeneous catalysis could address the appropriate characteristics of bio‐based carbonaceous catalysts. Generally, a heterogeneous catalytic reaction takes place through the following steps: (1) dispersion of the substrate from the bulk fluid to the pore entrance on the external catalyst surface; (2) diffusion of the substrate from the pore entrance into the internal catalyst pore; (3) adsorption of the substrate on the active catalyst site; (4) reaction of the substrate on the active site to generate a product; (5) desorption of the product from the active site; (6) diffusion of the product from the internal catalyst pore to the external surface of the catalyst; and (7) dispersion of the product from the external surface of catalyst into the bulk fluid [78].
The adsorption of the reactant on the active sites dispersed throughout the catalyst pores is the most important reaction step in heterogeneous catalysis. In case there is more than one reactant, competition in reactant adsorption on the active sites may occur. The concentration of each reactant on the active sites of the solid catalyst should be appropriate. The catalytic activity of the catalyst is based on the proportion of the reactants adsorbed on the active sites, and for each individual reaction, the chemical properties and functional groups of the bio‐based carbon catalyst should be designed to enable proper interactions with the adsorbed reactants. Diffusion steps 2 and 6 depend on the molecular size of the reactant and product as well as the pore morphology and catalyst particle size. Large catalyst particles diminish the reactant approaching the active sites inside the pores. So, the use of small catalyst particles can enhance the opportunity of a reactant to reach active positions within the pores. However, small catalyst particles cause a high pressure drop when used in a fixed‐bed reactor. To overcome this, the morphology of bio‐based carbon catalyst should be designed to have a higher external surface area such as a hollow cylindrical shape. Such information enables us to specifically control the catalyst characteristics and operating conditions of bio‐based
