(455c) Computational Kinetic Analysis of the Thermal Degradation of a Model Lignin Tetramer By Density Functional Theory

Lignin is a major component of lignocellulosic biomass and is one of the most abundant naturally occurring polymers in the world. Lignin could potentially be the most valuable component of lignocellulosic biomass due to its inherent aromatic composition, which is attractive for fuels and chemical applications. Thermolytic degradations of lignin yield complex and poorly predicted product spectra. The reaction mechanisms involved in the thermolysis of lignin remain obscure compared to the significant progress made towards cellulose and hemicellulose. In past studies, model lignin dimers have been theoretically and experimentally investigated to understand the pyrolysis behavior of specific lignin linkages and propose thermolysis pathways of the model dimers. Our goal is to expand on past investigations by further exploring the thermolysis of higher-order model compounds using computational density functional theory (DFT). We intend to use these well-defined model compounds to infer reaction pathways, kinetics, and mechanisms that are ultimately generalizable to whole lignin.

In this work, we investigated a tetrameric lignin model compound containing three prominent and critical lignin linkages, β-O-4, α-O-4, and β-5 (phenylcoumaran). These linkages represent approximately 70 % of all linkages in native lignin. Investigation of this model tetramer allows us to observe the relative decomposition behaviors of these linkages when in the vicinity of other linkages compared to their isolated model dimers. We performed kinetic analysis via DFT calculations using Gaussian16 with the M06-2X/6-311++G(d,p) level of theory. Reactants, products, and transition states were optimized, and frequency analysis was performed at 773 K (500 °C). As these molecules are complicated structures, a conformational search was performed to identify the lowest energy conformer of the reactants and products. Bond dissociation enthalpies (BDE) were calculated to identify major reaction pathways. Transition states were identified by the presence of a single imaginary frequency. We calculated the activation energies for each proposed reaction from relative energies between reactants and transition states, or the BDE in the absence of a transition state. The rate constants for individual reactions were calculated using transition state theory.