Biodiesel May Fuel The Future

Leftover vegetable oil is cheap...fast food restaurants in U.S. cities discard over 20 pounds of it per resident per year...but converting it to usable biodiesel requires an expensive chemical reaction.

Published: 28-Jan-2006

 When Rudolf Diesel developed his internal combustion engine in 1892, he envisioned that it would run not on petroleum, but on processed vegetable oil. The idea faltered because petroleum was much cheaper, but skyrocketing oil prices are calling new attention to the plant-based fuel now known as biodiesel.

The attractions of biodiesel extend well beyond the need for an alternative to limited petroleum supplies. Burning biodiesel results in little net addition of carbon dioxide to the atmosphere, since the oils were made from carbon dioxide recently extracted from the atmosphere by the plants themselves. Burning petroleum, on the other hand, transfers carbon from underground oil wells to the air and is one of the principal causes of global warming. Biodiesel also produces less of the pollutants responsible for acid rain and the rising burden of asthma in major cities. Leftover vegetable oil is cheap — fast food restaurants in U.S. cities discard over 20 pounds of it per resident per year — but converting it to usable biodiesel requires an expensive chemical reaction. A paper in the November 10 issue of Nature reports a marked improvement in biodiesel production that may help make it a practical alternative to petroleum.

Converting vegetable oil to biodiesel involves heating the oil in the presence of an acid catalyst. A catalyst is any chemical that facilitates a chemical reaction without itself being consumed; ideally, the catalyst could be retrieved after making biodiesel and used for the next batch. However, practical problems with catalysts have contributed to the high cost of biodiesel. Sulfuric acid is an effective catalyst, but is expensive to separate from the finished biodiesel. Solid acid catalysts can easily be removed from the completed reaction, but are expensive and tend to lose their activity rapidly.

Japanese chemists sought a way around these problems by attaching sulfuric acid to a solid base, so that it could be easily removed from finished biodiesel and reused. They heated the readily available sugars glucose and sucrose, which consist of oxygen atoms attached to carbon rings. The sugars combined to form an extended network of carbon and oxygen. Finally, this substance was treated with sulfuric acid, which attached itself to the molecular network in many places. The end product was an insoluble black powder which could be used to make biodiesel, easily recovered, and reused without detectable loss of activity.

If this and other technological improvements can make the price of biodiesel competitive with petroleum — which may happen soon if oil prices keep soaring — there is enough waste vegetable oil to replace about 5 percent of American petroleum consumption in the near future. But unlike petroleum, it would be easy to expand soybean production and make much more biodiesel if the economic incentives were in place. Stay tuned.

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