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] |
|
|
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. The resulting bio‐based activated carbon materials were highly porous with surface areas of 640–970 m2 g−1 depending on the biowaste source. Under optimal conditions, conversion of up to 94% could be achieved for esterification [120]. Slow reaction rates, the requirements of high methanol‐to‐oil molar ratios, and high reaction temperatures are, however, weaknesses of the sulfonated carbon catalyst for biodiesel production [122]. A further application with regard to the upgrading of bio‐oil via reactive distillation including water removal, catalytic esterification, and neutralization steps was carried out with p‐toluene sulfonic acid embedded on an activated carbon obtained by activation of sawdust pellets [123]. The catalyst was applied to the catalytic esterification in the reactive distillation procedure. After water removal from the oil, the esterification reaction took place at 80 °C under atmospheric pressure with the acid‐modified activated carbon catalyst and zeolite. The heating value of the upgraded bio‐oil obtained from fast pyrolysis was improved from 14.80 to 23.13 MJ kg−1 and was ascribed to the presence of higher ester and acetic acid contents in the oil according to the reaction in Figure 2.7. The properties of the upgraded oil were consistent with the diesel standard. Interestingly, sulfonated activated carbon catalysts have the potential for bio‐jet fuel production through co‐pyrolysis of biomass and plastic waste [124]. Prior to the sulfonation step, an activated corncob was converted by microwave‐assisted carbonization. The sulfonation temperature was a major factor that influenced the number of SO3H groups on the catalyst. These active groups benefited the co‐pyrolysis oil yield of nearly 98% at 500 °C for 15 minutes. The obtained oil consisted of aromatic and C9–C16 alkanes, which was qualified as a bio‐jet fuel.
A drawback of the sulfonated bio‐based activated carbon catalyst is that its preparation process is environmentally unfriendly owing to the heating of carbon materials in concentrated sulfuric acid for an extended time. Apart from sulfuric acid, tribasic potassium phosphate (K3PO4) was recently used to activate carbon materials. The K3PO4 impregnated on the activated carbon catalyst benefited the biodiesel production from waste cooking oil. With 5% catalyst loading, the biodiesel yield was 98% in a 12 : 1 methanol‐to‐oil molar ratio at 60 °C for four hours. Only 20% reduction in biodiesel yield was observed when the catalyst was used for five reaction cycles [125].
