Write about the synthesis and advantages of biodiesel.

Biodiesel is a mixture of mono-alkyl (methyl or ethyl) esters of long-chain fatty acids. It is made from vegetable oils (jatropha, karanja, soybean, palm, used cooking oil) or animal fats.

Synthesis – transesterification: the oil (a triglyceride) reacts with methanol in the presence of a base catalyst (NaOH or KOH, about 1%) at about 60–65 °C.

\( \underset{\text{triglyceride}}{\mathrm{CH_2OCOR}\,/\,\mathrm{CHOCOR}\,/\,\mathrm{CH_2OCOR}} + 3\,\mathrm{CH_3OH} \xrightarrow{\text{NaOH}} 3\,\underset{\text{biodiesel (FAME)}}{\mathrm{RCOOCH_3}} + \underset{\text{glycerol}}{\mathrm{CH_2OH\,CHOH\,CH_2OH}} \)

The glycerol layer is separated. The ester layer is washed and dried to give biodiesel. Acid catalysts or lipase enzymes can also be used.

Advantages

• It is renewable and biodegradable, and it is non-toxic.
• It is almost free of sulphur and aromatics, so it gives very little SO₂. CO, unburnt hydrocarbons and particulate emissions are also lower.
• It is nearly carbon neutral, because the plants absorb the CO₂ that is released.
• It has a high cetane number and good lubricity. It can be used in existing diesel engines, pure or blended (B20).
• Its flash point is high, so it is safe to store and transport. It also reduces dependence on imported petroleum.

Limitations: it gels in cold weather, NO_x emissions are slightly higher, it costs more, and it competes with food crops for land.