Oleh: adiriyadhi | 27 Februari 2008



Biodiesel is defined as the mono-alkyl esters of fatty acids derived from vegetable oils or animal fats. In simple terms, biodiesel is the product you get when a vegetable oil or animal fat is chemically reacted with an alcohol to produce a new compound that is known as a fatty acid alkyl ester. A catalyst such as sodium or potassium hydroxide is required. Glycerol is produced as a byproduct. The approximate proportions of the reaction are:

100 lbs of oil + 10 lbs of methanol → 100 lbs of biodiesel + 10 lbs of glycerol

Biodiesel can also be made from other feedstocks:

  1. Other vegetable oils such as corn oil, canola (an edible variety of rapeseed) oil, cottonseed oil, mustard oil, palm oil, etc.
  2. Restaurant waste oils such as frying oils
  3. Animal fats such as beef tallow or pork lard
  4. Trap grease (from restaurant grease traps), float grease (from waste water treatment plants), etc.

Transesterification is the process of reacting a triglyceride molecule with an excess of alcohol in the presence of a catalyst (KOH, NaOH, NaOCN3, etc.) to produce glycerin and fatty esters. The mixture of fatty esters produced by this reaction is known as biodiesel.

In the first step of the reaction, the NaOH reacts with methanol in an acid base reaction. The products of this first step of the reaction are a very strong base, sodium methoxide, and water. In the second step, the sodium methoxide breaks the glycerine section from the fatty acid section. The separation of the glycerine portion leads to the formation of three methyl esters (the biodiesel) and glycerol. The NaOH is regenerated as a product in the reaction. The biodiesel and glycerol are immiscible and will separate to form two layers. The glycerol layer will also contain NaOH and excess methanol. The separation of the biodiesel and glycerol layer is fortuitous in that we can easily separate and isolate our biodiesel product from the remaining product mixture.

The following procedure is for synthesizing a biodiesel mini-batch from 100% pure unused vegetable oil. This method can easily be modified for other oils such as canola, olive, soybean peanut etc. You may bring an oil of your choice from home.

1. Warm up 10 mL of 100% pure vegetable oil to about 60°C in a 100 mL beaker. Warming the oil up is not necessary, but increases the reaction rate.
2. Transfer about 2 mL of sodium methoxide solution (be sure the solution is well mixed – should appear cloudy) to a 50 mL beaker with a magnetic stirrer. Stirring gently, add the warm oil. Cover with watch glass and turn up stirrer to position 7 or 8. Stir for about 30 minutes.
3. Transfer the contents of the beaker into a 15 mL plastic centrifuge tube. The mixture will separate into two different layers. The glycerol will sink to the bottom, and the methyl ester (biodiesel) will float to the top. Allow the mixture to sit for about 15 minutes, and then place it in a centrifuge and spin for another 5 minutes (don’t forget to counterbalance the centrifuge). If the layers have not separated continue to centrifuge for another 5 minutes.
4. Using a transfer pipet, carefully draw off the top layer of biodiesel. Make sure not to get glycerol (bottom darker layer) in the biodiesel.
5. Use the IR, NMR and GCMS to identify your products.

For the IR spectrum, your instructor will show you how to operate the machine. Look for presence of a carbonyl group and OH group (due to methanol or glycerol impurity).

For the NMR spectrum, add 3-4 drops of your sample to an NMR tube and then add 0.7 mL of CDCl3 (deuterochloroform, a common NMR solvent). On the library there is an NMR spectrum of pure vegetable oil. Compare your spectrum to this to determine if you produced biodiesel.

For the mass spectroscopy, place 4 mL of methylene chloride and 1 drop of your product into the special mass spec vial (provided by your instructor). Note the number on the vial. Cap and return the vial to your instructor. Your sample will be analyzed by the autosampler within the next two days.

ADI RIYADHI ( http://chemistrywan.blogspot.com )

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