Investigation into the Pyrolysis Bond Dissociation Enthalpies (BDEs) of a Model Lignin Oligomer Using Density Functional Theory (DFT)

Pyrolysis is a promising technology for converting lignocellulosic biomass into chemicals, materials, and fuels. Understanding the fundamental reactions and mechanisms during the fast pyrolysis of lignocellulosic biomass is essential for improving the efficiency of this technology. Investigations of lignin fast pyrolysis reactions and mechanisms have thus far lagged relative to cellulose and hemicellulose and have largely been focused on lignin model dimers and monomers. These studies provide valuable information about the reaction tendencies of individual linkages; however, the complex and varying nature of lignin interunit linkages does not allow direct extrapolation to larger lignin structures. In this study, we computationally investigate homolytic bond scission reactions of a larger, synthesizable model lignin oligomer, containing three major lignin linkages, using density functional theory. The bond dissociation enthalpies of the model oligomer showed the trend for the four lowest BDEs C_α–O (β-5) < C_α–C_β (β-5) < C_α–O (α-O-4) < C_β–O (β-O-4). Our results show that the general trends identified in smaller dimer molecules are maintained while the magnitude of the BDEs is increased in the model oligomer. This work is an important first step in developing a library of reaction information for various lignin substructures.