Predicting lignin depolymerization yields from quantifiable properties using fractionated biorefinery lignins

Authors: PHONGPREECHA, THANAPHONG - Michigan State University; HOOL, NICHOLAS - Michigan State University; Stoklosa, Ryan; KLETT, ADAM - Clemson University; FOSTER, CLIFF - Michigan State University; BHALLA, ADITYA - Michigan State University; HOLMES, DANIEL - Michigan State University; THIES, MARK - Clemson University; HODGE, DAVID - Michigan State University

Submitted to: Green Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2017
Publication Date: 11/7/2017
Publication URL: http://handle.nal.usda.gov/10113/5852201
Citation: Phongpreecha, T., Hool, N.C., Stoklosa, R.J., Klett, A.S., Foster, C.E., Bhalla, A., Holmes, D., Thies, M.C., Hodge, D.B. 2017. Predicting lignin depolymerization yields from quantifiable properties using fractionated biorefinery lignins. Green Chemistry. 19(21):5131-5143.

Interpretive Summary: As more cellulosic biorefineries producing biofuels become operational, there exists a unique opportunity to diversify the co-product portfolio to include aromatic compounds that can serve as a bio-based chemical alternative to petroleum based aromatics. Not only does this serve as a means to produce renewable chemicals and materials, but also improves process economics. The source of the aromatic compounds originate from lignin, an aromatic compound which is the second most abundant natural biopolymer available in plant cell walls after cellulose. Currently, the chemical functionality of lignin is underutilized with lignin primarily used as a low-value solid fuel in present operations. This research attempts to clarify how lignin can be deconstructed to generate aromatic monomers by taking into account the origin of the lignin and processing history that tend to have a great impact on the structure, properties, and suitability of the lignin for a target application. Hardwood lignin was generated by pH-based fractionation of alkaline pretreatment liquor. The lignin recovered at specific pH intervals was characterized to identify certain properties (e.g. molecular weight, inter-unit chemical linkages, etc.). Additionally, each lignin fraction was depolymerized by three different approaches to determine aromatic monomer yields. A model was developed to try to compare the lignin properties to the achieved aromatic monomer yields by the various depolymerization processes. This model was experimentally validated through quantitative experiments using nuclear magnetic resonance (NMR) spectroscopy. Overall, the research identified lignin pretreatment and isolation strategies to maximize the achievable aromatic monomer yield.

Technical Abstract: Lignin depolymerization to aromatic monomers with high yields and selectivity is essential for the economic feasibility of many lignin-valorization strategies within integrated biorefining processes. Importantly, the quality and properties of the lignin source play an essential role in impacting the conversion chemistry, yet this relationship between lignin properties and lignin susceptibility to depolymerization is not well established. In this study, we quantitatively demonstrate how the detrimental effect of a pretreatment process on the properties of lignins, particularly beta-O-4 content, limit high yields of aromatic monomers using three lignin depolymerization approaches: thioacidolysis, hydrogenolysis, and oxidation. Through pH-based fractionation of alkali-solubilized lignin from hybrid poplar, this study demonstrates that the properties of lignin, namely beta-O-4 linkages, phenolic hydroxyl groups, molecular weight, and S/G ratios exhibit strong correlations with each other even after pretreatment. Furthermore, the differences in these properties lead to discernible trends in aromatic monomer yields using the three depolymerization techniques. Based on the interdependency of alkali lignin properties and its susceptibility to depolymerization, a model for the prediction of monomer yields was developed and validated for depolymerization by quantitative thioacidolysis. These results highlight the importance of the lignin properties for their suitability for an ether-cleaving depolymerization process, since the theoretical monomer yields grows as a second order function of the beta-O-4 content. Therefore, this research encourages and provides a reference tool for future studies to identify new methods for lignin-first biomass pretreatment and lignin valorization that emphasize preservation of lignin qualities, apart from focusing on optimization of reaction conditions and catalyst selection.