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2.5.1.2 Alkali Catalysis
ОглавлениеThe use of bases in transesterification is probably the most common practice of conversion of compatible feedstock (low in FFA and moisture) into esters. The conversion involves formation of an alkoxide ion from the base and the alcohol, which then targets the triglyceride (for example), attacking the carbonyl carbon, and forming a tetrahedral shaped intermediate. These compounds then undergo reaction with an alcohol molecule, undergoing a structural rearrangement during the process to give off an ester molecule and leading to the formation of a diglyceride [2]. This process will now repeat itself twice to finally yield glycerol as a by‐product along with 2 mol of alkyl ester [28], as shown in Figure 2.1. The process cannot tolerate even traces of FFA and moisture except for KOH, which is why it is favored over other bases. The process can also include homogeneous and heterogeneous catalysts, with doped basic groups being more stable compared to acidic groups [1, 6]. Many edible and nonedible oils (including waste oils) have been converted into biodiesel through this process; a select few of which are summarily presented in Table 2.2.
Table 2.2 Traditional catalyzed conversions using acids, bases, enzymes, or other catalysts.
Source: Modified from Ref. [2].
| Feedstock | Catalyst used | Reaction conditions | Yield/conversion (%) | References | ||||
|---|---|---|---|---|---|---|---|---|
| Reaction temperature (°C) | Reaction time (h) | Catalyst concentration (% w/w) | Alcohol:oil (ratio or wt%) | Agitation speed (rpm) | ||||
| Acid‐catalyzed conversion | ||||||||
| Castor oil | H2SO4 | 50 | 1 | 1 | 20 : 1 | 700 | 90.83 | [4] |
| Waste cooking oil | H2SO4 | 60 | 3 | 5 | 12 : 1 | 800 | 95.376 | [3] |
| Mahua oil | Sulfonated Delonix regia char | 50 | 1 | 4 | 6 : 1 | 1000 | 97.04 | [5] |
| Castor oil | Sulfonated MFL char | 60 | 1.5 | 7 | 70 | 950 | 92 | [16] |
| Base‐catalyzed conversion | ||||||||
| Waste cooking oil | KOH | 50 | 1.67 | 0.75 | 9 : 1 | — | 90 | [20] |
| Waste cooking oil | CaO | 75 | 1 | 5 | 9.8 : 1 | 450 | 96.6 | [21] |
| Waste cooking oil | CaO | 65 | 3 | 7.5 | 15 : 1 | 1200 | 90 | [22] |
| Waste cooking oil | Calcined egg shells | 65 | 5.5 | 3.5 | 22.5 : 1 | 600 | 91 | [23] |
| Enzyme‐catalyzed conversion | ||||||||
| Waste cooking oil | Lipase | 50 | 10 | 6 | 20 | — | 94 | [24] |
| Jatropha oil | Lipase | 40 | 8 | 0.5 | 4 : 1 | — | 71 | [25] |
| Other novel catalysts for conversion | ||||||||
| Waste cooking oil | Fe(II)‐doped anthill | 60 | 1.5 | 1.2 | 6 : 1 | — | 99.73 | [15] |
| Rubber seed oil | Fe(II)‐doped Delonix regia char | 40 | 15 | 5 | 3 : 1 | 500 | 96.31 | [26] |
| Two‐step catalyzed conversion | ||||||||
| Waste cooking oil | H2SO4 | 65 | 3 | 1 | 3 : 7 | 400 | 21.5 | [27] |
| NaOH | 50 | 3 | 1 | 3 : 7 | 400 | 90.6 | ||
| Karanja oil | Sulfonated Delonix regia char | 50 | 0.75 | 3 | 12 : 1 | 900 | 99.86 | [6] |
| KOH‐doped Delonix regia char | 60 | 1.5 | 4 | 6 : 1 | 700 | 99.39 |
