One of the most frustrating aspects of biofuels is the stark contrast between what exists in nature and what you can put in the tank.
Plants are rich in carbohydrates, a group of organic compounds based on carbon and water, itself a combination of hydrogen and oxygen. These carbohydrates take the form of chains of thousands of sugar units; each unit contains six carbon atoms and a similar number of oxygen atoms.
An ideal fuel, on the other hand, should lack oxygen. Its molecules should also be small, that is, they should contain few carbon atoms. Creating an efficient fuel from plants thus presents a headache.
From the Economist