(24e) Ethanol-Water Solutions at Elevated Temperatures for Isolating Ultraclean Corn Stover Lignins

Corn is the most abundant crop in America, yet the early push for realizing the energy potential of the non-food portion of the plant via cellulosic bioethanol has largely stalled in recent years. The current high cost of production has kept it from becoming competitive with fossil fuels or even traditional modes of corn ethanol production. By extracting value from the lignin co-product, cellulosic bioethanol could help supplant the renewables market without affecting the supply of valuable feed corn.

The dispersity and complexity of lignin has long presented a challenge to valorization, though if an effective method for molecular weight fractionation can be employed, this could be turned into an opportunity. Considering one particular pair of applications, polyurethane foams and carbon fibers, we can demonstrate the effectiveness of our ALPHA fractionation and purification process by transforming raw bulk lignin into precursors for these different applications.

Taking raw alkaline pretreated corn stover lignin containing almost 11% sugar and 3% ash, we were able to isolate high molecular weight lignin fractions containing less than 0.05% sugar and 0.05% ash, while simultaneously producing lignin an order of magnitude lower in molecular weight that served as an effective precursor for polyurethane foams. All this was accomplished using only ethanol-water solutions at mild temperature conditions (60-75℃). This approach was also applied to a lignin derived from hybrid poplar trees, and yielded similar results. Due to the difference in properties between the corn stover and hybrid poplar lignin and the softwood lignin that has been the subject of this technique previously, changes had to be made to the ALPHA process to account for the abundance and properties of the present impurities. This new approach to ALPHA enabled this significant reduction in impurities, while presenting interesting insight into the molecular weight fractionation behavior of both the corn stover and hybrid poplar lignin.

This ALPHA processing scheme relies on shifting the composition of the ethanol-water-lignin system to alter the solution strength. First, to selectively remove the impurities before precipitating out the lignin of controlled molecular weights. What we observe is that the highest molecular weight lignin does not precipitate out of solution during the first decrease of solvent strength, but instead much later when the solvent has already become weaker. The contribution of chemical functionality to this behavior is being investigated by ³¹P NMR, and will give insight into why this behavior is occurring and how we may be able to leverage it further.

Ultimately, the goal of creating high quality precursors for value added applications will come down to optimizing the properties. Through these adaptations to the ALPHA process, we have achieved some of the best corn-stover-based carbon fiber properties ever reported in literature as well as polyurethane foams that exceed ASTM standards even with very high lignin substitution rates.