Using carbon dioxide as a reversible method of controlling pH eliminates the need for separation and purification of the liquid after biomass pretreatment and before two other major steps used in biofuel production—saccharification and fermentation.

Pretreatment is the most expensive part of the biofuels production process. If you count the whole production cycle, pretreatment is second only to the cost of growing and obtaining the feedstock itself.

—Seema Singh, director of Biomass Pretreatment at JBEI, and corresponding author

According to a preliminary economic analysis reported in the study, a CO₂-enhanced process could lower production costs by 50 to 65 percent compared with conventional ionic liquid-based pretreatment methods.

Much of the appeal of using carbon dioxide gas to neutralize the ionic liquid is the ease with which the technique can be integrated into existing industrial operations. This solution is also relatively non-toxic compared to other common industrial gases or pH-adjustment techniques.

The application and removal of pressurized gas in an industrial setting are things we’ve been doing for more than a century. This technique fits into an already robust and reliable industrial system.

—senior author Blake Simmons, chief science and technology officer and vice president for Deconstruction at JBEI

The ionic liquids used for pretreatment at JBEI are typically highly alkaline and must be washed away so that they do not interfere with the enzymes and microbes used in the latter stages of biofuels production. The enzymes are needed to release the sugars from the slurry of cellulose and hemicellulose after pretreatment (saccharification). The bacteria produce the biofuel via fermentation of the sugars.

The study authors pointed out that microbes generate carbon dioxide as a byproduct of fermentation, so harnessing that gas for use in the pretreatment phase leads to an even greener source of energy.

Incorporating gaseous CO₂ in this process means there’s no need for a neutralization step, and the pH can be switched on a dime by the addition or release of CO₂. When the pH adjustment is reversible, it makes the overall process more efficient because you can repeat the pretreatment cycle several times. And it costs less because now you can do everything in one reactor instead of three.

—Blake Simmons

For several decades, researchers have been working to reduce the amount of energy and handling needed for each stage of this process. Some focus on engineering enzymes and bacteria so that they can withstand the exposure to ionic liquids, while this study focuses on neutralizing the ionic liquid so it won’t harm the enzymes and microbes.

When CO₂ is absorbed in water, the resulting chemical reactions increase the liquid’s acidity.

For their experiments, the researchers screened 15 types of ionic liquid at various concentrations. Of the ionic liquids, the scientists determined that cholinium lysinate, an ionic liquid formed by mixing the amino acids choline and lysine, was most compatible with commercially available enzyme mixtures and fermentation mixtures. They then conducted tests with various concentrations and pressures of carbon dioxide.

Applying up to 145 pounds per square inch of carbon dioxide to the system shifted the pH to a range that was optimal for the enzymes and microbes. This enabled the researchers to get more than 83% of the theoretical yield of ethanol from the glucose initially present in biomass.

The researchers said that this process could soon be ready for implementation in ethanol production.

The next steps are to adapt this to the production of “drop-in” advanced biofuels that can directly replace blendstock for today’s automotive, diesel and aviation fuels, the researchers said.

The study lead author is Jian Sun, a postdoctoral researcher at JBEI. Other co-authors are N.V.S.N. Murthy Konda, Jian Shi, Ramakrishnan Parthasarathi, Tanmoy Dutta, Feng Xu, and Corinne Scown.

JBEI is supported by DOE’s Office of Science.

Resources

• Jian Sun, Suryanarayana Konda, Jian Shi, Ramakrishnan Parthasarathi, Tanmoy Dutta, Feng Xu, Corinne D. Scown, Blake Simmons and Seema Singh (2016) “CO₂ enabled process consolidation for the production of cellulosic ethanol in bionic liquids” Energy Environ. Sci. doi: 10.1039/C6EE00913A