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biochar characteristics. Of the various functionalized catalysts, the sulfonated carbon‐based catalyst is among the most competent for biodiesel production [109]. Effects of the different forms of sulfonating precursors modified on raw biochar support were investigated. Biochar sulfonated with fuming sulfuric acid had a superior acid density over that sulfonated with concentrated sulfuric acid, resulting in a higher activity in the transesterification of vegetable oil. Further modification of biochar with KOH before sulfonating with fuming sulfuric acid could maximize the catalytic activity for transesterification as KOH ascribed to an enhanced porosity, surface area, and acid density of the catalyst [6]. Carbonization of biomass followed by sulfonation resulted in catalysts containing SO₃H, COOH, and OH groups, in which the number of SO₃H groups significantly influence the catalytic activity for biodiesel production [110]. Leaching of SO₃H groups caused the deactivation of the catalyst. The efficiency of esterification between oleic acid and ethanol calculated from the reduction of the reactant acidity reached 98.40% when the time to introduce the SO₃H groups into the bamboo‐derived catalyst was increased from 60 to 120 minutes [111]. Thus, not only the type of sulfonating precursor but also the sulfonating conditions affect the catalytic efficiency.

More information on biodiesel yield produced in the presence of various heterogeneous catalysts is summarized in Table 2.4. In comparison to non‐biomass‐derived catalysts mentioned in this discussion, not only the catalysts produced from carbon‐rich biomass but also from calcium carbonate‐rich biomass have the potential to be competitive with commercial alkali and metal oxide catalysts in biodiesel production. Even though biomass‐derived catalysts showed a slightly lower biodiesel yield, paying more attention to research in biodiesel production with bio‐based catalysts will shortly achieve better results in terms of product yield, catalyst stability, environmental friendliness and cost‐efficiency.

      2.5.3 Biomass‐Derived Activated Carbon

Biomass‐derived activated carbons are conventionally produced from diverse renewable sources via carbonization and activation. The sequence of processes in activated carbon production from biomass causes a considerable increase in the porosity and specific surface area. Other unique properties of activated carbon comprise thermal resistance, stability in both acidic and basic environment, and the possibility to tailor both its physical and surface chemical properties. To satisfy the requirements for catalyzing a specific reaction, activated carbons can be treated with KOH or CaO via wet impregnation. The stability of such immobilized catalysts, however, is relatively low. To improve the performance of activated carbon, chemical treatment with oxidizing acids such as SO₃H, H₃PO₄, or HNO₃ is an interesting choice [119–121]. In view of the catalytic potential, sulfonated carbon catalysts are widely attractive in reactions such as hydrolysis, esterification, and transesterification. Sulfonated bio‐based carbon catalysts can be prepared by direct sulfonation or sulfonation via reductive alkylation/arylation reactions.

Figure 2.6 Esterification of fatty acid.

Table 2.4 Comparison of various catalysts for biodiesel production.

Catalyst	Feedstock	Reaction conditions	Yield (%)	References
Alcohol:Oil	Catalyst amount (wt%)	Temperature (°C)	Time
La2O3/CaO	Jatropha oil	25 : 1	3	160	3 h	98.76	[112]
Li/CaO	Jatropha oil	12 : 1	5	65	1–2 h	>99	[113]
ZnO‐TiO2‐Nd2O3/ZrO2	Soybean oil	5.7 : 1	—	195	44 min	99	[104]
/SnO2	Waste cooking oil	15 : 1	3	150	3 h	92.3	[105]
Bio‐based carbon modified with Ni and Na2SiO3	Soybean oil	9 : 1	7	65	100 min	98.1	[114]
Sulfonated modified coconut meal residual	Waste palm oil	12 : 1	5	65–70	10 h	92.7	[115]
Sulfonated modified carbonized coconut shell	Palm oil	30 : 1	6	60	6 h	88.25	[116]
Sulfonated woody biochar	Canola oil	30 : 1	7	315	3 h	48.1	[117]
Sulfonated carbonized bamboo	Oleic acid	7 : 1	6	90	2 h	98.4	[111]
Calcium oxide from eggshell	Waste cooking oil	6 : 1	5.8	Room temperature	11 h	97	[118]

Sulfonated activated carbons from various biomass resources were tested for the production of biodiesel through esterification or transesterification. Various kinds of biomass wastes were activated with H₃PO₄ prior to carbonization and subsequently sulfonated through arylation of 4‐benzenediazonium.

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