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A simple, effective and relatively inexpensive technique can remove lignin from plant material used to make biofuels, which may drive down the cost of biofuel production.

Lignin, nature’s way of protecting plant cell walls, is difficult to break down or remove from plant materials called “biomass,” such as the non-edible parts of the corn plant. However, to reach the energy rich cellulose used to make biofuels, lignin needs to end up removed.

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“Finding inexpensive ways to remove lignin is one of the largest barriers to producing cost-effective biofuels,” said Ezinne Achinivu, a Ph.D. student in chemical and biomolecular engineering at North Carolina State University and lead author of a study describing the new technique. “And our approach is very promising.”

The researchers began by making a number of liquid salts called “protic ionic liquids” or PILs. These PILs are fairly inexpensive to prepare, because they consist of a mix of an acid, such as acetic acid (more commonly known as vinegar), and a base (a chemical class of materials called amines). As part of the pretreatment process, one of the PILs mixes with biomass and then ends up heated and stirred. The lignin dissolves into the PIL, leaving the cellulose behind as a solid. The cellulose, which is now much easier to process, then easily filters from the mixture for use in the next biofuel production steps.

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The remaining PIL-lignin liquid mixture can then heat up to distill (or vaporize) the PIL, leaving the lignin behind as a black powder. The vapors from the PIL then end up collected and cooled to recover the liquid PIL so it can be re-used. The lignin is also valuable, because the researchers can then use it to manufacture polymers or other chemical products which could supplement the cost of running the biofuel production facility.

“This PIL-based technique can be easily scaled up and is likely to be both more energy efficient and less expensive than existing biomass pretreatment techniques for removing lignin,” Achinivu said.

The researchers are working to apply the technique to wood and other biomass feedstock materials, as well as to better understand and fine-tune the interactions between the PILs and lignin. “If we can better understand how the PIL dissolves the lignin, we can make the process even more efficient by using less energy while extracting more lignin,” Achinivu said.

The paper, “Lignin Extraction from Biomass with Protic Ionic Liquids,” is online in the journal Green Chemistry.

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