Making use of Cellulose

 
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How do we get from A to B?

The process used to get from natural hydrocarbons to ethene has been outlined in the trails for 9.2.1. But how can we get from cellulose to ethene? Cellulose is a solid that is reluctant to dissolve in anything, unlike crude oil fractions which can easily be pumped into a cracker. Degrading it chemically would take very severe chemical treatment and a lot of heat and pressure - the amount of energy used to break it down would destroy any environmental benefit of the 'greener' source material.

Instead of such a process, we get a little help from the original and most numerous inhabitants of our planet - bacteria.

Cellulose to Glucose

The first stage of conversion of cellulose to ethene is the breakdown of cellulose to glucose.

Pretreatment is done to break down the macroscopic structure of cellulosic materials, which are usually things like grass cuttings, wood chips, or sugarcane trash. Soaking in caustic soda (industrial sodium hydroxide) or hot water swells the polymer matrix, allowing enzymes to penetrate to break up the cellulose chains and helping to separate the cellulose from lignin.

The breakdown of cellulose is then done by a family of enzymes called cellulases.

The three enzyme-catalysed reactions that break down cellulose to glucose are shown below.


Endoglucanase - chops randomly in the middle of polymer chains


Exoglucanase - chops a glucose dimer (cellobiose) off one end of the polymer chain

Beta-Glucosidase - converts cellobiose to glucose

These enzymes are found in bacteria that live in the stomachs of cows and termites, and in some fungi . One of these species of fungi, called Trichoderma reesei has been bred in the laboratory to produce more effective cellulases.