Tackling cellulose, lignin in biofuel systems

Tackling cellulose, lignin in biofuel systems

New enzyme digests cellulose faster than current cellulases; PILs aid removal of lignin from biomass.

RESEARCHERS at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have discovered that an enzyme from a microorganism first found on the Kamchatka Peninsula in Russia can digest cellulose almost twice as fast as the current leading component cellulase enzyme on the market.

If the enzyme continues to perform well in larger tests, it could help drive down the price of making lignocellulosic fuels, from ethanol to other biofuels that can be dropped into existing infrastructure, NREL said. A paper reporting this finding appeared recently in the journal Science.

C. bescii, first found in heated freshwater pools, secretes the cellulase CelA, which has the complex arrangement of two catalytic domains separated by linker peptides and cellulose binding modules.

NREL researchers put CelA to the test and found that it produced more sugars than the most abundant cellulase in the leading commercial mixtures, Cel7A, when acting on Avicel, which is an industry standard to test cellulose degradation. They found that CelA not only can digest cellulose in the more common surface removal, but it also creates cavities in the material, which leads to greater synergy with more conventional cellulases, resulting in higher sugar release.

The bacteria that secrete CelA thrive in temperatures of 167-194 degrees F (75-90 degrees C).

"Microorganisms and cellulases operating at such high temperatures have several biotechnological advantages," said NREL scientist Yannick Bomble, one of the paper's authors.

"CelA is the most efficient single cellulase we've ever studied — by a large margin," Bomble said. "It is an amazingly complex enzyme, combining two catalytic domains with three binding modules. The fact that it has two complementary catalytic domains working in concert most likely makes it such a good cellulose degrader."

Most commercial operations use enzyme cocktails — a combination of 15-20 different enzymes — to turn plant material into the sugars that are valuable to the biofuel industry. In most of these cocktails, Cel7A does the largest amount of work.

CelA was discovered 15 years ago, but until this recent work, all that was known about this complex protein was its general architecture and that it had the ability to degrade cellulose.

If an enzyme can produce sugars more efficiently, it means lower costs for the enzyme cocktail, which is a major cost driver in the process of converting biomass into fuel.

 

Simple technique

Meanwhile, researchers at North Carolina State University have developed a simple, effective and relatively inexpensive technique for removing lignin from the plant material used to make biofuels.

Lignin is difficult to break down or remove from plant materials such as the non-edible parts of the corn plant. However, that lignin needs to be extracted in order to reach the energy-rich cellulose that is used to make biofuels.

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

The researchers began by making a number of liquid salts called "protic ionic liquids" (PILs) by mixing together an acid, such as acetic acid, and a base, such as an amine.

As part of the pretreatment process, one of the PILs is mixed with biomass and then 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, is then easily filtered from the mixture for use in the next biofuel production steps, the announcement said.

The remaining PIL-lignin liquid mixture can then be heated to distill (or vaporize) the PIL, leaving the lignin behind as a black powder. The vapors from the PIL are collected and cooled to recover the liquid PIL so it can be reused.

The lignin is also valuable because it can be used to manufacture polymers or other chemical products that could supplement the cost of running the biofuel production facility, the university said.

"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 currently 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.

The paper, "Lignin Extraction from Biomass with Protic Ionic Liquids," was published online in the journal Green Chemistry.

Volume:86 Issue:02

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