Lignocellulosic biorefineries frequently employ a corn-stover feedstock to produce their bioethanol product. However, valorization of the accompanying, underutilized lignin byproduct will be required if these biorefineries are to achieve economic viability. Unfortunately, the heterogenous nature and impure state of the recovered bulk lignin render it difficult to directly use, particularly for higher-value applications. We are therefore investigating the Aqueous Lignin Purification with Hot Agents (ALPHA) process, which requires only hot aqueous ethanol solutions as the solvent system, for fractionating and purifying bulk lignins for desired, final-product applications. In contrast to previous work with softwood kraft lignin, modifications had to be made to ALPHA due to the nature and quantities of the impurities present in corn stover lignin. These modifications have resulted in the conversion of bulk, alkaline-pretreated corn stover lignin with a high impurities content (&gt;13% ash and sugar combined) into lignin fractions containing less than 0.05% ash and sugars, respectively. Similar results have been achieved with lignins derived from hybrid poplar. To our knowledge, these are the highest purity levels achieved to date with such lignins. Furthermore, the molecular weights of the resultant fractions can be varied by an order of magnitude during the ALPHA purification process. Such control of the properties of lignin is essential for applications such as polyurethane foams and carbon fibers, because of the impact on the finished product. By using the ALPHA process, lignin (polymer)-rich, liquid phases of controlled molecular weight (MW) can be precipitated from lignin–ethanol–water–solutions via water addition, resulting in both cleaned and fractionated lignins as described above. However, the fractionation behavior is unexpected, with the highest MW lignin precipitating not with the first water addition, but instead later in the process when water addition has significantly displaced the solvent from its peak strength. 31P-NMR is being used to investigate the extent to which lignin chemical functionality contributes to this unexpected behavior. With the lignin molecular weights and purities from this work, significant increases in the strengths of corn-stover-based carbon fibers can be achieved. Furthermore, ASTM-standard polyurethane foams with high polyol-for-lignin substitution levels can be made.

Ultraclean corn stover lignins of controlled molecular weight from hot lignin–ethanol–water solutions

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