The Chemistry of Biodiesel - Part 1
Fats and Oils
Biodiesel is made from vegetable oils and animals fats. Technically, the difference between an oil and a
fat is that an oil is liquid at room temperature while a fat is solid. However, both fats and oils, whether
from animals or plants, have the same basic structure: a “backbone” composed of a glycerin molecule
(also called glycerol) attached to three “tails” composed of fatty acids. This molecule is known as a
triglyceride.
There is a wide variety of oils and fats, and they differ only in the make-up of their fatty acid tails.
Common vegetable oils such as soybean oil, canola oil, or peanut oil, are actually mixtures of
triglycerides with different types of fatty acid tails. As we will see, biodiesel fuel properties vary with the
make-up of these fatty acid tails. The composition of a triglyceride is presented below.
Note that the glycerin backbone is composed for 3 carbons. The fatty acid tail can be composed of as
little as a half-dozen carbons to as long as a couple dozen carbons. As triglyceride molecules interact
with each other, the fatty acid tails tend to “tangle”. This increases viscosity (thickness). In fats, this
interaction increases to the point of solidifying the fat at room temperature. Triglycerides burn well in a
diesel engine, but their viscosity causes problems in fuel lines and fuel injectors. To make a good diesel
fuel, viscosity must be reduced.
Biodiesel
Biodiesel fuel is essentially the individual fatty acid tails (with a small modification) separated from the
glycerin backbone. Pictured below is a typical biodiesel molecule. Note how similar it is to a typical
diesel fuel molecule.
Typical biodiesel molecule
Typical diesel molecule
Note how both are basically a chain of about 16 carbons surrounded by hydrogens. Biodiesel differs only
in a structure at one end known as an “ester.” The ester has a single carbon beyond the oxygen, so it is
known as a “methyl ester.” This is why biodiesel is known chemically as “fatty acid methyl ester” or
FAME. The biodiesel molecule, freed from the glycerin backbone, is less viscous and burns very well in a
diesel engine. Unlike diesel, the ester end makes it much less toxic and much more biodegradable.
Making Biodiesel
To make biodiesel (FAME) we must separate the fatty acid tail from the triglyceride and then add the
ester end. This actually occurs in essentially one step, known as “trans-esterification.” A catalyst,
typically sodium hydroxide (NaOH) or potassium hydroxide (KOH), is mixed with methanol (MeOH) to
produce a mixture known as methoxide. (Remember that OH in an inorganic salt is a base, while OH in
an organic molecule is an alcohol, a mild acid). Together, this mixture can remove the fatty acid tail from
a triglyceride molecule and add an ester end to it. The methanol is consumed, but the catalyst is not.
Heat is generally added to accelerate the reaction.
For simplicity, R is used to represent the fatty acid carbon chain. In addition to producing biodiesel, the
reaction liberates glycerol. These two molecules have different polarities and different densities, so they
separate after formation. Biodiesel, with its long hydrocarbon chain, is relatively non-polar and less
dense, so it tends to float to the top. Glycerin, with its three alcohol (OH) groups, is much more polar
and dense, and sinks to the bottom. The glycerin can then be drained away, leaving the biodiesel.
In this image of a simple biodiesel reaction conducted in a 2L bottle, you can clearly see the dark glycerin
at the bottom and the lighter biodiesel floating on top.
Stoichiometry
How much of the reactants do we need to add? And how much biodiesel and glycerin will we produce?
We use stoichiometry to answer this question.
Looking back at the biodiesel reaction equation above, it is clear that one triglyceride molecule releases
three fatty acid tails, which become three molecules of biodiesel. So a whole bunch of triglyceride
molecules (a “mole”*) produces three-times as many molecules (3 moles) of biodiesel. Also note that it
takes three methanol (MeOH) molecules to convert one triglyceride to three biodiesel molecules. Each
triglyceride molecule will ultimately become one glycerin molecule. Thus one mole of triglyceride needs
three moles of MeOH to make three moles of FAME and one mole of glycerin.
1 mole oil + 3 moles MeOH = 3 moles FAME + 1 mole glycerin
Note, however, that this reaction is reversible – the glycerin and FAME can combine to recreate MeOH
and triglyceride. Ultimately, the reaction reaches equilibrium where you have some of each of the
molecules present. This is a problem if our goal is to make as much FAME as possible. To push the
reaction to make more FAME, we can increase the amount of MeOH we add. Typically, we double the
amount of MeOH to 6 moles for every 1 mole of triglyceride.
1 mole oil + 6 moles MeOH = 3 moles FAME + 1 mole glycerin + 3 moles MeOH
This is typically sufficient to push the reaction essentially all the way to the right, with only trace
quantities of oil left. The downside is that there are now 3 moles of unreacted MeOH mixed with our
products. Getting the MeOH out is a challenge we’ll address later.
What about the catalyst? How much should we add? Because the catalyst is not consumed, we can’t use
stoichiometry to answer this question. Instead, experience demonstrates how much additional catalyst
speeds up the reaction. In this case, experience shows we should add about ! moles of catalyst for every
one mole of oil. Remember that the catalyst remains in the products and will have to be removed.
1 mole oil + 6 moles MeOH + ! moles catalyst = 3 moles FAME + 1 mole glycerin
Determining the weight of each reactant to add simply requires knowing the molecular weight (how
much a mole of each substance weighs). We’ll take that step when you produce bench-scale biodiesel.
(* recall that through the work of Avagadro and others, we now define a “whole bunch of molecules” as 6.02 x 1023
molecules, or a “mole”.)