To convert biomass, pretreat with ammonia

MICHIGAN STATE (US) — Researchers have identified a potential pretreatment method that can make plant cellulose five times more digestible by enzymes that convert it into ethanol.

Presently, ethanol or other biofuels can only be produced in usable quantities if the biomass—corn leaves, stalks, or switchgrass—is pretreated with costly, potentially toxic chemicals in an energy-intensive process.

The new discovery, reported in the Journal of the American Chemical Society, could change that.

“What we’ve discovered is something like a cost-effective switch or a lever,” says Shishir Chundawat, postdoctoral researcher working with Bruce Dale, professor of chemical engineering and materials science at Michigan State University.

“By using an ammonia-based solvent, we were able to pull a lever and flip the entire cellulose crystal from one structure to another, one that’s much easier to break down.”

Biomass is a desirable renewable energy source because fermentable sugars within the cellulose network of plant cells can be extracted with enzymes and then converted into ethanol, but one of difficulties in the complicated is that cellulose tends to naturally orient itself into a sheet-like network of highly ordered, densely packed molecules.

These sheets stack upon themselves and bond tightly together due to strong interactions between molecules—somewhat like sheets of chicken wire stacked together and secured by loops of baling wire.

The stacking and bonding arrangement prevents enzymes from directly attacking most of the individual cellulose molecules and isolating the sugar chains within them.

During pretreatment, the water was removed and the ratio of ammonia was increased. As a result, the “baling wire” was replaced with loose thread, which made it vulnerable to conversion into sugars.

The end result is a potentially less costly and less energy intensive pretreatment regimen that makes the cellulose five times easier to attack.

The research was supported by the Great Lakes Bioenergy Research Center, a partnership between the University of Wisconsin and Michigan State.

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