(560v) Engineering Lignin Transformation Mechanisms to Create Value-Added Products Using Atomistic Modeling

Tanzina Azad¹, Jonathan D. Schuler¹, Maria L. Auad^1,2, Thomas Elder³, Andrew J. Adamczyk^1,*

¹Department of Chemical Engineering, Auburn University, Auburn, AL, USA.

²Center for Polymer and Advanced Composites, Auburn University, Auburn, AL, USA.

³United States Department of Agriculture (USDA) Forest Service, Southern Research Station, Auburn, AL, USA.

^*Corresponding author aja0056@auburn.edu

Among the three polymers that form plant cells (i.e., cellulose, lignin, and hemicellulose), lignin is the second most abundant natural polymer and one of the least utilized biomass sources. Lignin’s high natural abundance, carbon content, and functionalized nature make it a promising candidate for targeted valorization to fuels and polymer composites, as well as serving as a building block for various value-added products of renewable origin.¹ Due to the nature of its structure, lignin has the potential to be one of the largest renewable sources for aromatic compounds to be used in the chemical industry with an approximate market of $130 billion USD.² Despite this demonstrated benefit, lignin is still largely utilized as a low-grade fuel because its degradation reaction mechanisms and energetics under various reactor process conditions, as well as basic atomic-level structure, are still not fully understood. Therefore, computational modeling of lignin at the atomistic scale has the potential to provide new insights to help guide experimental reaction design and process optimization towards a tailored valorization process with high selectivity and yield.

In this work, we are analyzing the transformation of lignin macromolecules by cleaving the b-O-4 bond homolytically, which simulates pyrolysis reactor conditions. Among the three monolignols that are precursors for the biosynthesis of lignin (i.e., p-hydroxyphenyl (H), Guaiacyl (G), and Syringyl (S) units), only G- and S-units are analyzed in our study. Though the concept of lignin modeling is not recent, very few studies are available that explored lignin molecules with a large number of monolignol units. In this study, we have modeled lignin structures comprised of 20 repeating monolignol units. We have focused upon conformer analysis and Bond Dissociation Energetics (BDEs) calculations for various lignin macromolecules. Our methods include both classical molecular mechanics and quantum chemical methods for advanced conformational sampling and Density Functional Theory (DFT) calculations, respectively. We have developed a novel and robust method for lignin macromolecule conformational sampling, which maps the conformational energy landscape efficiently and provides multiple low energy structures that are then used to determine the corresponding DFT reaction energetics of degradation.

We have also constructed a lignin conformer library which is a product of our developed conformational sampling method. This structural library is very useful for further computational studies and cross validation with experiments. This presentation will discuss the advanced conformation sampling method in the context of various commonly used force fields parameters, and provide specific guidance on force field parameter selection for lowest energy conformer identification. Our computational models correlate very well with available experimental data on structure and energetics. Lastly, the presented results demonstrate how theoretical investigations are currently impacting new materials discovery and synthesis understanding in the development of novel polymers and advanced composites.

1. Roberto Rinaldi, Robin Jastrzebski, Matthew T. Clough, John Ralph, Marco Kennema, Pieter C. A. Bruijnincx, Bert M. Weckhuysen, Paving the way for Lignin valorization: Recent advances in Bioengineering, Biorefining and Catalysis, Chem. Int. Ed. 2016, 55, 8164 – 8215.
2. Frost & Sullivan. (2014, 7 May) Full Speed Ahead for the Lignin Market with High-Value Opportunities as early as 2017 [Press Release]. Retrieved from http://www.frost.com/