Biodiesel Synthesis
Objective :
Learn to synthesize and analyze biodiesel fuel from vegetable oil.
Background: (you may write the summary of this in your notebook but cannot cut and
paste this section!)
In this experiment you will synthesize biodiesel fuel from vegetable oil. Oils (called
triglycerides or triacylglycerols) have a glycerine backbone joined by ester linkages to
three fatty acid chains. The chemical structure below shows the different areas within a
typical oil (triglyceride) molecule with one of the three ester linkages circled. The fatty
acid portions may vary in lengh between C12 and C18.
Triglyceride molecule
Ester linkage
O
CH2
O
O
CH
O
CH2
O
O
Glyerine
portion
Fatty acid portion
Different oils (canola, olive, soybean, etc.) differ in the percent composition of fatty acid
chain lengths. The reaction to form biodiesel from a triglyceride is known as a
transesterification reaction. Transesterification is the process of transforming one type of
ester into another type of ester. The reaction is catalyzed by the presence of the strong
base, NaOH.
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 general reaction is shown on the next
page.
triglyceride
+
methanol
3 methyl ester
+
glycerol
Biodiesel is a fuel, which means that it produces energy through combustion with oxygen
(as does regular diesel). The combustion reaction is shown below:
O
CH3O
+ 26 O2
18 CO2 + 18 H2O + Energy
Note that in the combustion reaction each carbon contained in biodiesel is converted to
carbon dioxide. As each carbon is oxidized to carbon dioxide it will release about 850 kJ
per mole. The more carbons, the more energy produced upon combustion.
Vegetable oil itself is also a fuel and can undergo combustion with oxygen. If this is so why
do we need to convert the vegetable oil (triglycerides) to the methyl esters—why not just
burn vegetable oil? The reason is due to the viscosity of the triglycerides. Viscosity is the
internal friction or stickiness of a liquid. A viscous material is ‘thicker’ and does not flow
quickly (i.e., syrup is a viscous liquid). See http://www.brevardbiodiesel.org/viscosity.html
for a comparison. The viscosity of vegetable oil will present a problem in that its stickier
nature will prevent it from flowing easily through the fuel pump systems of an engine.
Since viscosity is generally inversely proportional to temperature, at lower temperatures
the viscosity problem is enhanced. Unlike the triglycerides, methyl esters (biodiesel) are
less viscous and will easily flow through the fuel system of an engine, though there is some
controversy over the type and percentage of biodiesel that can be used in a standard diesel
engine due to issues with gelling of the fuel.
Procedure :
Part 1.
Note: 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.
1. Warm up 10 mL of your oil sample to about 60°C in a 100 mL beaker. Warming the oil
up is not necessary, but increases the reaction rate. Do not over heat!
* The name of the reaction is “trans-esterification”. On the drawing below, circle the
part that is (are) esters in the oil molecule and box the part that is (are) esters in
biodiesel.
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. Stir under low heat for about
20 minutes. (Great time for part 2!)
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 10 minutes, and then
place it in a centrifuge and spin for another 3 minutes (don’t forget to counterbalance the
centrifuge). If the layers have not separated continue to centrifuge for another 3 minutes.
4. If you are instructed to prepare GCMS vial : Mix 4 mL of CH2Cl2 with one drop of
biodiesel (Make sure not to get glycerol (bottom darker layer) in the biodiesel) in a
small test tube. Mix well. Obtain pre-labelled GCMS vial and fill it with the mixture.
Put used plastic pipets to regular garbage can and used glass pipets to ‘broken glass
deposit’ container.
Part 2.
Discuss with your group and pick one or two set of procedures you want to try. Write
down the actual procedures.
1. Test and rank the viscosity of liquid samples. – These will be located next to the sink.
Keep them there so it won’t get mixed up with the reagents from Part 1.
Prelab
Hand write the procedure including any data tables you will need during lab into
your lab notebook, and answer the following.
a. Look at http://www.biofuels.coop/ and write three facts from the website.
b. Print out the COMMON FATTY ACID table from
http://www.scientificpsychic.com/fitness/fattyacids.html and tape it on to the
notebook.
c. Vegetable oil is a mixture. Look up what’s in vegetable oil. Pick one that is on the
PL2 list. Write the type of oil and it’s chemical composition.
d. Why don’t we use the oil itself as fuel? (Think about what happens to oil in your
fridge or freezer)
e. You will be given few liquid samples. Come up with set procedures to test and
compare their viscosity. – You will be doing this in part 2 of this lab using what you
have in your drawer, so make the procedure relatively simple.
Postlab
Clearly report your results, any graphs, tables, and answers to your postlab
questions. Unless using excel, everything is handwritten. Include statements for any
problems/errors encountered, and of what you learned from this lab.
Questions:
1. Why did the two products of this experiment (glycerol and biodiesel) separate?
2. How many molecules of biodiesel are produced for each molecule of oil?
3. One argument for using biodiesel is that the net amount of CO2 released into the
atmosphere is claimed to be zero (or near zero). How can this be, given that the
combustion of biodiesel released CO2 (see reaction equation of previous page)?
4. What does it mean if a GC spectra has three peaks?
Summary:
Type of oil used
chemical name
viscosity results