The methoxide ion, –OCH3, is the active catalyst for the production of methyl esters. It is this chemical unit that attacks the triglyceride molecules and produces the methyl esters. It is regenerated at the end of each reaction step when a hydrogen ion is stripped from a nearby methanol molecule.
If ethanol is being used, then the corresponding catalyst is called ethoxide, –OCH2CH3.
Most small producers create the methoxide ions needed for the reaction by dissolving sodium hydroxide or potassium hydroxide in methanol. When this is done, the hydroxide splits apart, or dissociates, and then undergoes the following reaction.
- CH3OH + Na+ + –OH → Na+ + –OCH3 + H2O
This reaction creates, from the hydroxide catalyst, the methoxide ion needed for the biodiesel production reaction. Unfortunately, the reaction also creates a molecule of water. Water causes the formation of soap through a chemical reaction called saponification. When sodium or potassium hydroxide are used as catalysts, the soap production will be similar to what happens when oil or fat containing water is used.
Commercial producers prefer to get their methoxide from solutions of sodium (or potassium) methoxide already dissolved in methanol. Sodium methoxide, also known as sodium methylate, can be purchased as a 25% or 30% concentrate and is made with a water-free process so the catalyst does not contribute to soap formation.
While sodium methylate solutions appear to be more expensive than sodium hydroxide or potassium hydroxide, they can be less expensive in practice. First, since the solution contains methanol, it lessens the amount of methanol that must be added to the reaction. There are some savings from this. Second, it is usually possible to use less sodium methylate than the amount of sodium hydroxide needed to get the same extent of reaction. This is because of the loss of sodium hydroxide associated with soap formation. Finally, the increased soap formation from the use of a hydroxide catalyst causes a loss of yield that wastes a significant portion of the feedstock oil. Not only is the oil which goes to soap lost, but also some of the good biodiesel product is lost when the glycerin is removed at the end of the reaction. This is because soap increases the solubility of biodiesel in the glycerin phase.