Producing bioethanol without enzymes

Researchers from Denmark and Iraq have developed a technique to produce second-generation biofuel without the use of expensive enzymes. Their work has been published in the journal Applied Catalysis A.

Bioethanol is made from the remains of plants after other parts have been used as food or other agricultural products. It is seen as a strong potential substitute for fossil fuels - one that reduces CO₂ emissions - and countries like the United States and Brazil are far ahead when it comes to producing bioethanol from plant parts like corn or sugar cane.

But as corn cobs and sugar cane can also be used directly as food, this has resulted in some public resistance. A big challenge is therefore to become able to produce bioethanol from plant parts which cannot be used for food. The goal, says Professor Per Morgen of the University of Southern Denmark, is to produce bioethanol from cellulose.

“Cellulose is very difficult to break down, and therefore cannot directly be used as a food source,” said Professor Morgen. “Cellulose is found everywhere in nature in rich quantities, for example in the stems of the corn plant. If we can produce bioethanol from the corn stems and keep the corn cobs for food, we have come a long way.”

Cellulose is organised in long chains in the plant’s cell walls. There are various patented enzymes on the market that can break down cellulose into sugar, which then is used to produce bioethanol, but these are expensive to buy. So Professor Morgen’s Iraqi colleagues, from the University of Baghdad and Al-Muthana University, developed an acid which can perform the same role, made from treated rice husks.

The ashes from burnt rice husks have a high content of silicate. The scientists paired silicate particles with chlorosulfonic acid and this made the acid molecules attach themselves to the silicate compounds.

“The result was an entirely new molecule - the acid RHSO3H - which can replace the enzymes in the work of breaking down cellulose to sugar,” said Professor Morgen.

“The worldwide production of rice generates enormous amounts of rice husk and ashes from burning the husk, so this material is cheap and easy to get hold of.”

Professor Morgen noted that the method is environmentally friendly and accessible for all. The catalyst acid is made from readily available plant leftovers and it can be re-used many times. The bioethanol is produced from cellulosic plants that cannot otherwise be used for anything else. Importantly, the method cannot be patented.

It is hoped that the method will make the production of second-generation biofuel cheaper, thus attracting more producers and competition, which may bring the price down for consumers.