Brewing bioethanol in a single pot.

Brewing bioethanol in a single pot.

A more efficient way to produce biofuels from plants is possible by pretreating the woody material with a liquid salt before fermentation, report researchers in Japan who perfected the process in the lab. Yet, coming up with the usable, purified cellulose remains a big hurdle to the industry. 

This experimental method is unique because it pairs an enzyme-yeast unit with ionic liquids to convert the plants into the liquid fuel known as bioethanol. The successful trial yielded 90 percent ethanol and 82 percent of the ionic liquid was recovered.

The one-step, one-container procedure is outlined in the journal Green Chemistry.

Cellulose is the most abundant renewable material available. Plants, algae and some bacteria produce in excess of 100 billion metric tons per year. The non-food material is an agricultural waste material.

The ability to turn the unwanted cellulose into liquid fuel would be an important step toward reducing dependence on crude oil without using food crops.

It takes three steps to convert cellulose into the liquid fuel bioethanol. Step one treats cellulose and turns its rigid and ordered structure into more chemically accessible pieces. In step two, enzymes further break down cellulose into glucose, a sugar. Then, in step three, microorganisms such as yeast ferment the glucose to ethanol.

The new process takes a different approach. The cellulose is broken down with ionic liquids (step 1) then converted into ethanol (steps 2 and 3) with a yeast-enzyme pairing – all in a single pot.

This is a remarkable step forward because the enzyme, yeast and ionic liquid are together but don’t interfere with one another.

Ionic liquids are salts – a designation for chemicals made up of both a positively and a negatively charged component – that are liquid at low temperatures, unlike common salts such as kitchen salt. Bulky positive and negative groups that make up the ionic salts hinder their packing into a solid crystal. Thus, they stay liquid to much lower temperatures – even room temperature. Some ionic liquids have been shown to be good solvents for cellulose, which is otherwise very difficult to dissolve.

In this study, the authors attached cellulase – an enzyme that breaks down cellulose – to the outside of the yeast. By itself, this yeast-cellulase combination is not very effective at tearing apart the rigid and inaccessible cellulose structure. But, a winning recipe was found by combining a small amount of specific ionic liquids with the cellulase-yeast. The ionic liquid disrupts cellulose enough so that ethanol can be produced directly and efficiently from the pieces of cellulose.

This work shows some real promise, but as the authors point out, most cellulose is not pure. To turn biomass directly into bioethanol, this new one-pot process will need to either extract cellulose efficiently or convert the other natural materials that coexist with cellulose in the plants.