Model lignin oligomer pyrolysis: Coupled conformational and thermodynamic analysis of β-O-4′ bond cleavage
The abundance, carbon content, and functionalized nature of lignin make it a promising candidate for targeted valorization to fuels and polymer composites. While lignin modeling by the application of computational chemistry is an active area of research, electronic structure methods have been limited mainly to structures in the dimeric or trimeric range. In this study, we have modeled a lignin structure composed of 10 β-O-4′ linked guaiacyl (G) units, such that this work represents, to the best of our knowledge, the largest structure that has been examined to date using quantum mechanical calculations. As such, this work can provide information on a model, the size of which is more representative of the lignin polymer than has been previously reported. We have calculated bond dissociation enthalpy (BDE) for the homolytic cleavage reaction between each G unit in our model lignin oligomer, which occurs as one of the initial reactions during lignin pyrolysis. The objective of the current work was to determine how or if reactivity within the oligomer changes as a function of bond cleaving position within the chain. The methods used were classical molecular mechanics for conformational sampling and quantum mechanically based density functional theory (DFT) calculations. We have developed a novel and robust method for conformational sampling, which maps the conformational energy landscape efficiently and provides multiple low-energy structures that are then used to determine the BDE values by DFT. Our results for BDE calculations of lignin exhibit significant position dependence along the oligomer chain. To the best of our knowledge, we have reported for the first time the calculated standard thermodynamic properties including enthalpy of formation, heat capacity, entropy, and Gibbs free energy. Despite using a simplified model lignin oligomer structure, our calculated values for standard thermodynamic properties have a remarkable agreement with the experimental values.