An introduction to glycerol

Many years ago we went on a family holiday to York. Driving around the by-pass we happened to see a small scale manufacturer of biodiesel (from waste fat).  This was when biofuels were seen as the saviour of the planet, even by green groups. I stopped and had a chat with the woman in charge.  She asked me as a scientist what she should do with all the glycerol produced as part of the process.  I hadn’t a clue and suggested burning it (not such a great idea with hindsight).  This whole issue of waste glycerol disposal which Jeremy has asked me to write a guest blog on, raises some interesting ecological dilemmas.

Glycerol (also known as glycine) is a colourless viscous liquid with a boiling point of just under 300°C. Technically its an alcohol, with 3 alcohol groups on it, I would prefer to call it propan- 1,2,3-triol (see a) in figure 1)).

glycerol and family figure 1

Glycerol is becoming an increasing waste issue, since its a by-product of soap and biodiesel manufacture.  Chemically when triglycerides (fats or oils) are reacted with an alkali (figure 2. below)

Saponificacao

Glycerol is produced along with 3 long chain fatty acids (R is a chemists short hand for a chain of carbons and hydrogen atoms). The only difference chemically in making biodiesel is you add an alcohol to this mix as well.  This ends up bonded instead of the sodium to the fatty acid making an “ester”, which is chemically what biodiesel is.  About 10% of the reaction volume of the biodiesel reaction mixture ends up as glycerol.

There are essentially two options of how to deal with the glycerol glut.  Use the glycerol directly or make it into something useful.  Glycerol has in itself many uses.  Its used in food as a thickener and sweetener (E422).  It has various pharmaceutical uses (my Grandparents had some in bathroom cabinet).  Its used as component of the e-liquid in E-cigarettes.  Glycerol also now utilised by a company to produce electricity to power the formula E racing cars.  They have worked out a way to use it in diesel engines.

These above uses obviously mop a lot of glycerol up.  A more interesting proposition is to chemically modify the glycerol.  Reducing it to a diol with 2 rather than 3 alcohol groups present (b) in the figure 1, produces another set of compounds (either propan-1,3-diol (PDO) or propan-1,2-diol).  Propan-1,2-diol is used as antifreeze on aircraft wings and as lubricant for condoms, its also used to dissolve foodstuffs such as vanilla essence in. Both diols can be used to make plastics, PDO is regarded as more useful.

Another issue is how you convert the glycerol to the diols.  The traditional way is to use metal catalysts and loads of energy. However, increasingly the chemical industry is going biological.  This latter way uses either bacteria or their enzymes to convert glycerol to the diol by fermentation.  The advantages of enzymes are that they can be very specific and use a lot less energy.  I supervised a student project to try this since my bugs grew on glycerol (my idea was to make glycerol into electricity using bacteria). Unfortunately my bugs failed to make PDO from glycerol, possibly due the formation of a antibacterial intermediate called 3-HPA, or the presence of oxygen.  However in itself 3-HPA has a number of potential uses, not least as a potential food preservative.

Plenty of other groups have succeeded in using bugs to make PDO, Dupont are making it from glucose in the form of Corn Syrup.  This offers us the means of making plastics in a more environmentally friendly way and breaking our oil dependence.  Most plastics however are not biodegradable and a biological route only solves some of the problems with our plastic addiction, a bit like electric cars viz à viz oil based vehicles.

Very slightly modified guest post I did for Make Wealth History.

Neil

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