Biodiesel Chemistry Tutorial

• Biodiesel is a mixture of fatty acid esters.
• Biodiesel is also known as FAME (Fatty Acid Methyl Esters)
• Biodiesel can be synthesized from vegetable oils.
• Vegetable oils typically contain triglycerides.
• Triglycerides are converted to fatty acid methyl esters (biodiesel) in a transesterification reaction.
• Biodiesel is slightly less energy dense than diesel derived from crude oil.
• Biodiesel is a renewable fuel, diesel is a non-renewable fossil fuel.
• Diesel engines need slight modifications to run on biodiesel to prevent damage to rubber components such as fuel lines.¹

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Theory: What is biodiesel? Why can biodiesel be used as a fuel?

Diesel² is non-renewable fuel produced in oil refineries by the fractional distillation of crude oil.
The fraction collected with a boiling point between 200°C and 300°C is used for industrial and commercial heating as well as diesel fuel.
This fraction contains hydrocarbon chains from C₁₅ to C₁₈.
For example, the fraction used in diesel fuel might contain the hydrocarbon C₁₆H₃₄, hexadecane, as shown below:

Biodiesel, on the other hand, is a renewable fuel, produced by converting vegetable oils into methyl³ esters of long chain fatty acids.
An example of a methyl ester that could be found in biodiesel, C₁₅H₃₁COOCH₃, is shown below:

The vegetable oils such as palm oil, canola oil, rapeseed oil and soybean oil used to produce biodiesel are made up of triglycerides. An example of one triglyceride is shown below:

Vegetable oils are usually very viscous ("thick") because the long carbon chains on the triglyceride molecules tend to become tangled up. This increased viscosity compared to conventional diesel fuel causes problems in the fuel lines and fuel injectors of diesel engines. In order for a vegetable oil to be a good replacement for diesel in diesel engines, the viscosity of the vegetable oil needs to be reduced.
If each triglyceride molecule is broken up into three individiual fatty acid residues, then the viscosity of the resulting substance is decreased and is about the same as for conventional diesel.
The triglycerides are broken up into 3 individual carbon chains in a chemical reaction known as a transesterification reaction.⁴

In a base-catalysed transesterification reaction, the glycerol molecule is broken off the triglyceride molecule resulting in 3 fatty acid molecules (long chain carboxylic acids) which can then react with methanol to produce the methyl esters of each of the three long chain carboxylic acids:

Note that the base, sodium hydroxide (NaOH), used in this reaction is a catalyst, that is, it is not consumed during the reaction. Other catalysts which are commonly used are potassium hydroxide or sodium methanolate (sodium methoxide).

Also note that the reaction is in fact reversible. In order to drive the reaction forward an excess of methanol is used.

If the vegetable oil is kept dry⁵ and contains few free fatty acids, the reaction produces a yield of about 98%.

At the end of the reaction, the mixture contains the biodiesel (methyl esters) as well as glycerol⁶ which is a waste product.
Even though biodiesel's methyl ester molecules contain the polar ester functional group, the result of the very long non-polar hydrocarbon chains is to reduce the overall polarity of the molecule so that it can be considered to be non-polar. With only weak Van der Waal's forces (dispersion or London forces) acting between the biodiesel molecules, the long carbon chains do not pack together tightly, so the mixture is not very dense.
Glycerol, propane-1,2,3-triol, on the other hand has 3 polar hydroxyl (OH) function groups on a short carbon chain. Each OH group can form stronger hydrogen bonds to other OH functional groups resulting in a tighter packing arangement and hence a more dense liquid.

As a consequence, the non-polar methyl ester molecules making up the biodiesel do not mix with the polar glycerol molecules and the mixture of products will separate into two layers with the less dense biodiesel floating on top of the more dense glycerol layer.
This allows the desired biodiesel to be decanted off the unwanted glycerol.

There are other methods of producing biodiesel. One of these uses enzymes, lipases, to catalyse the conversion of vegetable oil to biodiesel. However, methanol cannot be used in this reaction because it results in the inactivation of the lipase catalyst after it has been used. However, if the methanol is replaced with methyl acetate (methyl ethanoate), the lipase catalyst remains active for several batches.

Biodiesel is not a pure substance, it will be a mixture of substances, most of which will be methyl esters of fatty acids. Diesel derived from crude oil is also not a pure substance but a mixture of different hydrocarbons.
This means that is not possible for us to write a simple balanced chemical equation for the combustion of diesel or biodiesel, and therefore it is not possible to measure the heat of combustion of diesel or biodiesel in units of kJ mol^-1.
In order to compare the energy content of diesel or biodiesel, we use one of the following:

• Specific Energy: energy per unit mass, for example kJ g^-1 or J kg^-1
• Energy Density: energy per unit of volume, for example J mL^-1 or kJ L^-1

Note that the given energy content of diesel (specific energy or energy density) is slightly more than that for biodiesel.

The given specific energy of biodiesel is 42 MJ kg^-1, that is, 1 kg of biodiesel has a heat content of 42 MJ.
While Industrial Chemists often use large amounts of substances, 1 kg of biodiesel is good for them, but Chemists, or students in the school laboratory, need to work with much smaller amounts so we need to be able to convert this to an energy content per gram.

1 MJ = 1 megajoules = 10⁶ J (or 1 000 000 J)
So, 42 MJ kg^-1 = 42 × 10⁶ J kg^-1

Since 1 kg = 1 kilogram = 10³ g (or 1 000 g)
Then 42 × 10⁶ J kg^-1 = 42 × 10⁶ J kg^-1 ÷ 10³ g kg^-1 = 42 × 10³ J g^-1
and 42 × 10³ J g^-1 = 42 × 10³ J g^-1 ÷ 10³ J kJ^-1 = 42 kJ g^-1

If we have 10 grams of biodiesel its heat content would be 10 g × 42 kJ g^-1 = 420 kJ

Synthesis of Biodiesel

Procedure to produce biodiesel in the school laboratory:

Procedure to separate biodiesel from glycerol:

1. Decant (gently pour off) the top layer containing the biodiesel into a small (100 mL) clean, dry, conical (Erlenmeyer) flask.

   Label this flask "top layer: biodiesel".

2. Tip the vial containing the remaining mixture on its side and position a pipette in the bottom of the mixture.

   Carefully remove this dense bottom layer only using the pipette, and place this into a clean, dry test tube.

   Label the test tube "bottom layer: glycerol solution".

   If, on standing, the solution in the test tube separates into 2 layers, use a pasteur pipette to remove and discard the top layer.

Sample observations:

Analysis of Biodiesel: Solubility

Confirm that the top layer, now contained in the conical (Erlenmeyer) flask, is biodiesel and that the bottom layer, now contained in a test tube, is the glycerol by determining their relative solubility in water.

Procedure:

1. Place 5 mL of water in each of two test tubes. Label one test tube "top layer" and label the other "bottom layer".
2. To the water in the test tube labelled "top layer" use a pasteur pipette to add a few drops of solution from the conical (Erlenmeyer) flask.

   Swirl gently. Observe.

3. To the water in the test tube labelled "bottom layer" use a pasteur pipette to add a few drops of solution from the test tube.

   Swirl gently. Observe.

Sample results:

Glycerol contains 3 polar OH functional groups which allow it to hydrogen bond with water molecules. Glycerol is therefore miscible with water (that is, it is soluble in water in all proportions).
Biodiesel, a mixture of methyl esters of long chain fatty acids, is effectively non-polar so it will not dissolve in water, that is, it is not miscible with water. Another way to say this is that it is immiscible with water.

Analysis of Biodiesel: Density

Confirm that the top layer, now contained in the conical (Erlenmeyer) flask, is more likely to be biodiesel than vegetable oil by measuring their relative densities.

Procedure:

1. Label one clean, dry, 10 mL measuring cylinder "top layer" and another one "vegetable oil".
2. Weigh each measuring cylinder.

   Record the mass.

3. Pipette 5.00 mL of the "top layer" solution from the conical flask into the measuring cylinder labelled "top layer".

   Weigh and record the mass.

4. Pipette 5.00 mL of the vegetable oil used in the experiment to produce the biodiesel into the measuring cylinder labelled "vegetable oil".

   Weigh and record the mass.

5. Calculate the density of each:

   density = mass of substance (grams) ÷ volume (millilitres)

Sample results:

The "top layer" (biodiesel) should be less dense than the vegetable oil used to make the biodiesel.

Analysis of Biodiesel: Heat of Combustion

Compare the energy released by combustion of the biodiesel produced above with that released by the combustion of conventional diesel.
Although we will be using very small amounts of diesel and biodiesel, the procedure is very similar to that for determining the heat of combustion of alcohols. Note that instead of using a spirit burner we will be using an evaporating basin and wick.

Procedure:

1. Record the mass of a clean, dry, evaporating basin.
2. Add 1 mL of the prepared biodiesel to the evaporating basin and record its total mass.
3. Calculate the mass of biodiesel in grams in the evaporating basin:

   mass(biodiesel) = mass(evaporating basin + biodiesel) - mass(evaporating basin)

4. Record the mass of a clean, dry, 50 mL beaker.
5. Add 15 mL of water to the beaker and record its combined mass.
6. Calculate the mass, in grams, of water in the beaker:

   mass(water) = mass(beaker + water) - mass(beaker)

7. Loosely fanfold a 5 cm piece of tissue to be used as a wick.
8. Position the tissue in the evaporating basin so that some of the tissue is touching the biodiesel in the bottom of the basin.
9. Place wire gauze on an iron ring attached to a retort stand (or ring stand).

   Adjust the height of the iron ring so it is as close to the evaporating basin as possible.

10. Position the beaker of water on top of the gauze on the iron ring.

    Record the initial temperature of the water in °C

11. Light the tissue wick and record the time.
12. When all the biodiesel has been combusted, record the time taken and the maximum final temperature the water reached.
13. Repeat the procedure using 1 mL of diesel instead of biodiesel (preferably in a fume cupboard).

Sample results:

Sample Calculations:

1. Heat released by combustion of fuel which is used to heat the water
   = mass(water) × specific heat capacity of water × (final temperature - initial temperature)
   specific heat capacity of water = 4.18 J °C^-1 g^-1
2. Heat released per gram of fuel = heat released by combustion of fuel (J) ÷ mass of fuel (g)

Biodiesel

1. energy released by combustion of 0.88 g biodiesel = 15.00 × 4.18 × (35-20) = 940.5 J
2. energy released per gram of biodiesel = energy released ÷ mass = 940.5 J ÷ 0.88 g = 1069 J g^-1

Diesel

1. energy released by combustion of 0.85 g diesel = 15.00 × 4.18 × (37-20) = 1065.9 J
2. energy released per gram of diesel = energy released ÷ mass = 1065.9 J ÷ 0.85 g = 1254 J g^-1

Combustion of diesel releases a little more energy per gram than the combustion of biodiesel, and the diesel combusts at a faster rate than biodiesel.

Footnotes: