Turning Polyethylene Waste into Fuel

Low density polyethylene, LDPE, is used to make many things we use everyday such as plastic milk containers, cling wrap, and plastic bags. LDPE can be recycled quite easily, but a lot of LDPE ends up as rubbish in land fills.
Chemists in India have developed a commercially viable way to turn LDPE into a liquid fuel by heating the LDPE waste to between 400 and 500^oC over a kaolin catalyst which causes the long chain polymer chains to break apart. This process is known as thermo-catalytic degradation: thermo means heat, the catalyst is kaolin, and degradation means breaking apart. This thermo-catalytic degradation of LDPE produces much smaller carbon-based molecules. Gas Chromatography was used to characterise these smaller molecules, and it was found that they were mainly alkanes and alkenes between 10 and 16 carbon atoms long. This makes the mixture very similar to that found in petrochemical fuels. For example, the hydrocarbons in gasoline (petrol) typically have a chain length of between 4 and 12 carbon atoms while diesel fuel typically contains hydrocarbons with a chain length between 8 and 21 carbon atoms.

The catalyst, kaolin, is a layered, aluminosilicate clay mineral with the formula Al₂Si₂O₅(OH)₄. It acts as a catalyst by providing a large surface on which the polymer molecules can sit in an orientation favourable to the degradation under heat.

Using the kaolin catalyst at 450^oC, the thermo-catalytic degradation of 1 kg of LDPE produced 700 g of liquid fuel.

Reference:
Achyut Kumar Panda, Raghubansh Kumar Singh. Thermo-catalytic degradation of low density polyethylene to liquid fuel over kaolin catalyst. International Journal of Environment and Waste Management, 2014; 13 (1): 104 DOI: 10.1504/IJEWM.2014.058803

Further Reading:
Polythene (polyethylene): Properties, Production and Uses 
Gas Chromatography 
Alkanes: properties and uses
Alkenes: properties and uses
Ethene (ethylene): properties and uses
Silicates: structure and formula

Suggested Study Questions:

1. Give the molecular structure for ethene (ethylene).
2. Write an equation showing how ethene (ethylene) molecules can be polymerized to form polythene (polyethylene).
3. Name the type of polymerization reaction being described by the equation in question 2.
4. Explain, using a diagram of the partial structure of polythene (polyethylene), what happens when polyethylene undergoes thermo-catalytic degradation.
5. Describe the differences in the structures of alkanes and alkenes.
6. Draw a straight chain alkane with 10 carbon atoms.
7. Draw a structural isomer of the molecule in question 6.
8. Draw a straight chain alkene with 10 carbon atoms.
9. How many structural isomers of the molecule in question 8 do you think there would be? Support your answer with the structural formula for each of these structural isomers.
10. Why do you not find short carbon chain alkanes, between 1 and 4 carbon atoms long, in the liquid petrochemical fuels like gasoline (petrol) and diesel?

Sepiolite

Sepiolite has been known since Roman times when it was used to filter and purify wine. No other mineral is known to absorb more water or other liquids as efficiently as sepiolite, which is why sepiolite is commonly used in cat litter. Sepiolite is also used to absorb liquid spillages, such as in oil spills, and odours and stabilise aqueous products like paints, resins and inks.
Sepiolite is an aluminosilicate clay mineral with a typical formula of Mg₄Si₆O₁₅(OH)₂·6H₂O.

Sepiolites absorb moisture by using tiny tunnels in the crystals. The elongated, needle-shaped sepiolite crystals pack very loosely into a lightweight porous material. The surface area ranges between 75 and 400 m²/g, meaning that 20g of mineral have an internal surface equivalent to that of a football court. This is why sepiolite can absorb 2.5 times its weight in water. The tunnels in the crystal structure along with the empty space between the needles form a capillary network through which liquids can easily flow deep inside the bulk where the molecules attach to the surface of the crystals.

A team of scientists from Spain and France has obtained, for the first time, single-crystal X-ray diffraction images of sepiolite, opening the path to industrial synthesis and further improvement of its properties. In synthetic form, sepiolite could bind food products and stabilise drugs, extending their shelf life and making sepiolite an edible product.

Reference
Manuel Sanchez del Rio, Emilia Garcia-Romero, Mercedes Suarez, Ivan da Silva, Luis Fuentes Montero, and Gema Martinez-Criado. Variability in sepiolite: Diffraction studies. American Mineralogist, 2011 DOI: 10.2138/am.2011.3761

Further Reading
Percentage Composition
Balancing Chemical Equations


Study Questions

1. Calculate the percentage composition of sepiolite, Mg₄Si₆O₁₅(OH)₂·6H₂O
2. Explain what the ·6H₂O part of the formula refers to.
3. Write a balanced chemical equation for the dehydration of hydrated sepiolite to form anhydrous sepiolite.
4. Calculate the maximum mass of water you could obtain from 1kg of hydrated sepiolite.
5. If the sepiolite in question 2 has a surface area of 200m²/g, what is the total surface area of the sample in question 4?
   
6. Use the equation in question 3 to explain why sepiolite is used in cat litter.
7. Sepiolite is an aluminosilicate mineral. Explain what is meant by the term aluminosilicate.
8. Sepiolite is sometimes referred to as a zeolite-like mineral. In what ways is sepiolite similar to a zeolite mineral?