Computational studies on lignin's pyrolysis using electronic structure methods have been limited mainly to dimeric or trimeric models. We have modeled two different lignin oligomers comprised of 10 monolignols (guaiacyl and syringyl, respectively) linked through 9 β-O-4′ bonds. Lignin models of this size are potentially more representative of the polymer in plant species such as gymnosperms and angiosperms. Therefore, these model structures have been used to examine representative soft and hardwood lignin behavior during the initial steps of pyrolysis. This work aims to determine if and how the reaction enthalpies of bond cleavage vary with positions within the chain using these oligomers. We followed a composite method using molecular mechanics-based conformational sampling and quantum mechanically based Density Functional Theory (DFT) calculations. Our key results show marked differences in bond dissociation enthalpies (BDE) with the position of β-O-4′ bonds. Additionally, we calculated standard thermodynamic properties, including enthalpy of formation, heat capacity, entropy, and Gibbs free energy for a wide range of temperatures relevant to pyrolysis conditions up to 1000 K. The prediction of these thermodynamic properties and the reaction enthalpies will benefit further computational studies and cross-validation with reaction kinetics experiments. Overall, the results demonstrate the utility of a better understanding of lignin pyrolysis for its effective valorization.

Revealing the position dependence of bond dissociation enthalpy for the homolytic cleavage of β-O-4 linkage using model lignin oligomers

ACS Fall 2021

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