Biodiesel has become quite popular over the past five years. That name has been tossed around so much that it’s become a bit of a “buzzword”. Biodiesel is simply liquid fuel that can be burned in a diesel engine, and which originates from renewable, naturally-occurring sources such as plants. Vegetable oil, in particular, is a popular feedstock for the biodiesel industry. A molecule of vegetable oil consists of two basic parts. In the center of the molecule you have a glycerol molecule. Glycerol is similar to ethylene glycol in structure, although it contains three alcohol groups instead of two. In vegetable oil, three long alkyl chains are attached to the glycerol alcohols by carboxylic acid linkers. Upon treatment of the vegetable oil with a mixture of lye and methanol, the carboxylic acids release their hold on the glycerol and are freed as their methyl esters. The reaction products from one molecule of vegetable oil are therefore one (three-pronged) molecule of glycerol and three long alkyl chains capped with methyl esters. The methyl esters are the components that are used for diesel fuel, and are decanted away from the glycerol. The glycerol is the problem; it’s currently an unwanted reaction product, and is usually disposed of by burning.
Glycerol finds some uses in cosmetics and foodstuffs, but the market for the compound is already severely depressed. As more and more biodiesel production comes online, we’re going to rapidly find ourselves swamped with more glycerol than we can easily handle. While incineration may seem like a convenient method of getting rid of the waste from biodiesel, it’s far from an ideal solution, for numerous reasons. It produces carbon dioxide, for starters – carbon dioxide is a strong greenhouse gas, and so biodiesel production with glycerol incineration actually hurts the environment twice: once when the glycerol is burned, and the second when the diesel is burned. Also, alcohol groups such as those present on the glycerol molecules are some of the most versatile and powerful “starting points” for organic synthesis. It seems an awful shame to waste such a versatile starting material simply by burning it. Thankfully, chemists have now taken an important step towards full utilization of the glycerol remnants from biodiesel production: they make alkane fuel out of it.
Alkanes are simply molecules that only contain carbon and hydrogen. Long alkane chains are useful as fuels; the chemical name for an 8-carbon alkane chain is “octane”, after all. Chemists already have a suitable reaction to synthesize long chain alkanes; it’s called the Fischer-Tropsch reaction, and relies on passing a mixture of hydrogen and carbon monoxide gas over a particular type of metal catalyst. What’s needed is a reaction to transform glycerol (which has carbons, oxygens, and hydrogens) into the mixture of hydrogen and carbon monoxide (which is called “syngas” by some branches of chemistry). That goal has now been reached. American scientists report in the journal Green Chemistry that they have designed a platinum / rhenium / carbon catalyst that (at elevated temperatures) converts a feed of glycerol into the desired syngas mixture.
This new catalyst also produces ethanol and acetone as intermediates during the reaction, which can be siphoned off and sold as they are valuable products in their own right. The conversion to the syngas is otherwise quite clean and it’s not even necessary to purify the syngas; it can be directly pumped into a second, Fischer-Tropsch, reaction vessel that is onsite with the glycerol reactor. This technology should spur the progress and growth of the biodiesel industry. Now that we can finally put all of the vegetable oil molecule towards a useful purpose and no longer have to worry about disposing of the glycerol, we can concentrate on ramping up the number of biodiesel manufacturing plants. The overall process has just become more attractive, economically speaking, and so this should encourage new investment and even more research / development.
The source of this article can be found at: http://www.rsc.org/Publishing/Journals/GC/article.asp?doi=b704476c