Theoretical Studies on the Bond Dissociation Enthalpy Values for β-O-4 Bond Cleavage in Model Lignin Oligomers Under Pyrolysis Conditions

Computational modeling of lignin at the atomistic scale has the potential to provide new and better insights for its tailored valorization process to build lignin efficient technology. In this work, we are analyzing the homolytic cleavage of β-O-4 bond in our model lignin oligomer under pyrolysis reactor conditions. Though lignin depolymerization has been studied over a century, system with a large number of monolignol units has not been studied extensively yet. Here, we have modeled lignin oligomer with structure size 10-mer for two different monoligols, Guaiacyl (G), and Syringyl (S) units), to investigate if/how bond dissociation energy varies as the β-O-4 bond is cleaved at different positions along the full oligomer chain. 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 conformational sampling, which provides a library of lignin conformers that are then used for the corresponding DFT calculations. In addition, we have predicted major thermodynamic properties for a wide range of temperature which could be useful for further computational studies and cross validation with experiments.