How is biodiesel synthesised by transesterification and what are its advantages as a green fuel?

In one line

Biodiesel is a mixture of fatty acid methyl (or ethyl) esters made by transesterification of vegetable oils or animal fats with methanol in the presence of a base catalyst, giving glycerol as a by-product.

Explained simply

Vegetable oil can burn in a diesel engine, but it is too thick (viscous) and leaves gum. Transesterification "cuts" each big oil molecule into three smaller ester molecules, which flow like diesel. It is like breaking a heavy three-person bench into three light chairs. The leftover backbone is glycerol, a useful product for soaps and cosmetics.

How it works

1. Raw materials: oils such as jatropha, karanja (pongamia), soybean, rapeseed, palm, or waste cooking oil; methanol; catalyst NaOH or KOH (or sodium methoxide).
2. Dissolve the catalyst in methanol to form methoxide.
3. Mix with oil (typically 6:1 molar ratio methanol : oil, excess to drive the equilibrium forward) and heat to about 60 °C (below methanol's boiling point, 65 °C) with stirring for about 1 hour.
4. Allow to settle: the lower layer is glycerol; the upper layer is crude biodiesel.
5. Separate, wash biodiesel with warm water to remove catalyst, soap and methanol, then dry.
6. If oil has high free fatty acids (waste oil), first do acid-catalysed esterification (H₂SO₄) to avoid soap formation.

Key formula

\( \text{Triglyceride} + 3\,\mathrm{CH_3OH} \xrightarrow{\text{NaOH/KOH, }\sim 60\,^\circ\text{C}} 3\,\mathrm{RCOOCH_3} + \text{Glycerol} \)

\( \mathrm{CH_2OCOR} \)–\( \mathrm{CHOCOR} \)–\( \mathrm{CH_2OCOR} + 3\mathrm{CH_3OH} \rightarrow 3\mathrm{RCOOCH_3} + \mathrm{CH_2OH{-}CHOH{-}CH_2OH} \)

• R = long hydrocarbon chain of the fatty acid (C₁₅–C₁₇ commonly)
• RCOOCH₃ = fatty acid methyl ester (FAME), i.e., biodiesel

Quick example

Advantages:

• Renewable and biodegradable; non-toxic.
• Nearly carbon-neutral over its life cycle, since plants absorb CO₂ while growing.
• Almost no sulphur, so very little SO₂; lower CO, unburnt hydrocarbons and particulate (soot) emissions, because it contains about 11% oxygen.
• Higher flash point than diesel, so safer to store and transport.
• Good lubricity; reduces engine wear. Can be blended with petro-diesel (B5, B20) or used pure (B100) in compatible engines.
• Uses waste cooking oil and non-edible oils (jatropha, karanja), and the by-product glycerol has value.

Limitations: slightly lower energy content, possible small increase in NO_x, poorer cold-flow properties, and food-versus-fuel concerns with edible oils.

Common mistakes

• Calling biodiesel "vegetable oil"; it is the transesterified ester.
• Writing water as the by-product; it is glycerol.
• Using an acid catalyst for low-FFA oil and calling it standard; base catalysis is standard and faster.

How MU asks it

"Explain the synthesis of biodiesel with reaction. State its advantages" (5 marks). (Biodiesel is also a lab experiment in this course, where % atom economy is calculated.) For full marks: reaction with triglyceride structure, conditions (catalyst, ratio, temperature), separation of glycerol, and 5–6 advantages.