Toward Native Hardwood Lignin Pyrolysis: Insights into Reaction Energetics from Density Functional Theory

Lignin is one of the three structural components of lignocellulosic biomass and the only renewable source for sustainably producing aromatic chemicals. Despite lignin's promise for targeted valorization to fine chemicals, fuels, and polymer composites, a major roadblock is an ambiguity associated with its molecular structure. Due to the lack of an established molecular structure, all types of computational studies from reaction mechanism to reaction energetics are performed with model lignin compounds. Among several thermochemical conversion techniques, lignin pyrolysis has been an active research area for both experimental and computational investigations. Historically, computational studies on the pyrolysis of lignin have mainly been limited to dimeric or trimeric models using electronic structure methods. Very recently, model oligomers up to the size of decamers have been studied with density functional theory (DFT) calculations. While these studies used very simplified models to study lignin's pyrolysis behavior, theoretical investigations on a more realistic lignin structure are warranted to advance its state-of-the-art. To address this, we have modeled a native hardwood lignin polymer consisting of 20 repeating units, including all three monolignols, i.e., guaiacyl (G), syringyl (S), and p-hydroxy coumaryl (H) units, and multiple types of linkages, i.e., β-O-4, 4-O-5, β–β, and β-5. This more realistic molecular model for the wild-type poplar lignin is based on a proposed lignin structure reported in a previous experimental study. The initial stage in lignin pyrolysis involves the homolytic cleavage of β-O-4 linkages present in lignin. The activation energy for homolysis of this linkage is considered to be slightly greater than the bond dissociation enthalpy (BDE) required to cleave it. We used a composite method using molecular mechanics-based conformational sampling and quantum mechanically based density functional theory (DFT) calculations to determine the reaction energetics for this reaction, which indicates the activation energy required for the early stage of lignin pyrolysis. In addition, we calculated standard thermodynamic properties for all species, including enthalpy of formation, heat capacity, entropy, and Gibbs free energy of formation as a function of temperature. This study provides the reaction energetics and standard thermodynamic quantities that could be used in kinetic and reactor modeling for biomass conversion. Additionally, these predictions would be particularly helpful in advanced-generation biorefinery, where hardwoods can be used as a potential feedstock. Moreover, the predictions reported in this study will benefit further computational studies and cross-validation with pyrolysis experiments, ultimately contributing to solving the puzzle of the structural ambiguity of lignin.